Exchanges 69

Transcription

Exchanges 69

No. 69 (Vol 20, No.1) July 2016
Special Issue: the Kuroshio Current and Extension System
w
Top: the semi-monthly Kuroshio/KE paths in years 1993-2015 based on satellite altimeter data, revealing the stable vs. unstable dynamical
states of the Kuroshio/KE system. Bottom: the regional bottom topography (in color) and surface dynamical topography (in white contours)
surrounding the Kuroshio and KE (courtesy of Bo Qiu, University of Hawaii).
CLIVAR Ocean and Climate: Variability, Predictability and Change is the World Climate
Research Programme’s core project on the Ocean-Atmosphere System
Editorial
Nico Caltabiano
International CLIVAR Project Office, Southampton,
UK
The scientific community working on the Kuroshio Current and
Extension has been very active for many years. This shows
the importance of this system and its impact on climate,
storm tracks, and ocean ecosystems. This issue of Exchanges
highlights some of the latest findings on the Kuroshio System
that were presented at another successful workshop organised
by the CLIVAR Ocean Model Development Panel, jointly with
the Climate Dynamics Panel, the Pacific Region Panel and
other partners. On behalf of CLIVAR, I would to like to thank
the Japan Agency for Marine-Earth Science and Technology
(JAMSTEC) for hosting the event, and to US CLIVAR and the
Intergovernmental Oceanographic Commission (IOC) SubCommission for the Western Pacific (WESTPAC) for financial
support to participants.
This is going to be a very important year for CLIVAR. All
CLIVAR panels, the Scientific Steering Group (SSG) and the
International CLIVAR Project Office (ICPO) are fully engaged
in the organization of the CLIVAR Open Science Conference
that will take place in a few months, on 18-25 September, in
Qingdao, China. The success of the conference can already
be shown by its numbers: 936 abstracts from 750 authors
(66 countries) have been submitted and reviewed. Accepted
contributions will be presented as orals or posters in 14
sessions. Twelve town halls will be organised that will provide
a discussion forum for several communities. Jointly with the
conference, CLIVAR, together with the Young Earth System
Scientists (YESS) Community, is organising the Early Career
Scientists Symposium (ECSS). For this event alone, we
had 239 applicants, and 150 of those have been selectedto
attend the Symposium, based on the quality of their abstracts
submitted to the OS . It will be an great opportunity for the
future generation of climate scientists to not only learn more
about CLIVAR activities, but also discuss and provide input on
what they see as important climate science topics that CLIVAR
should address. All of this in a friendly environment that will
also allow them to network with colleagues from around the
world. More details about the OSC and ECSS, including the
latest programme, can be found at the conference website:
www.clivar2016.org
In the Project Office, we had some recent staff changes. This
is the first issue of Exchanges after I took over Anna Pirani’s
role as editor. Anna, who was an essential part of the ICPO
since she joined in 2007, has taken on a new position as
Head of the IPCC WG1 Technical Support Unit. Anna and her
determination to improve CLIVAR as a whole will be missed, but
I am sure we will continue to work together through CLIVAR’s
contribution to the IPCC process. I would like to wish every bit
of success to Anna in her new role. And Jing Li has joined the
ICPO team in Qingdao. Jing comes from the United Nations
Development Programme (UNDP) in Beijing, where she has
been Programme Assistant for Global Environmental Facility
(GEF) Small Grants Programme (SGP) for over 6 years. Before
that Jing worked at International Cooperation Department in
State Oceanic Administration (SOA) of China for over 2 years,
and worked as an Environmental Impact Assessment (EIA)
Engineer for 1 years at the Third institute of oceanography/
SOA, Xiamen, China. Jing is an environmentalist with a master
degree in Environmental Science, specialized in Environmental
Economics. Welcome Jing!
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CLIVAR Exchanges No. 69, Vol. 20, No. 1, July 2016
CLIVAR/JAMSTEC
Workshop on the
Kuroshio Current
and Extension
System: Theory,
Observations, and
Ocean Climate
Modelling-The
Workshop Overview
and Outcomes
Yoshiki Komuro1, Gokhan
Danabasoglu2, Simon Marsland3,
Xiaopei Lin4, Shoshiro Minobe5, Anna
Pirani6, Tatsuo Suzuki1, Ichiro Yasuda7
1. Japan Agency for Marine-Earth Science and
Technology, Japan
2. National Center for Atmospheric Research, USA
3. CSIRO Oceans and Atmosphere, Australia
4. Physical Oceanography Laboratory/CIMSST,
Ocean University of China and Qingdao National
Laboratory for Marine Science and Technology,
China
5. Hokkaido University, Japan
6. Université Paris Saclay, Saint-Aubin, France
and The Abdus Salam International Centre for
Theoretical Physics, Trieste, Italy
7. University of Tokyo, Japan
Introduction
The “CLIVAR/JAMSTEC Workshop on the Kuroshio Current
and Extension System: Theory, Observations, and Ocean
Climate Modelling” was held in Yokohama, Japan, January
12-13, 2016. The workshop was hosted by the Japan Agency
for Marine-Earth Science and Technology (JAMSTEC), and
jointly organized with three CLIVAR panels (Ocean Model
Development Panel (OMDP), Pacific Regional Panel, and
Climate Dynamics Panel) and the WESTPAC project on AirSea Interaction in the Kuroshio Extension and its Climate
Impact (AIKEC). International scientists contributed expertise
of theoretical, observational, and numerical modelling studies
of the Kuroshio. Seventy two participants from 9 countries
presented 34 talks and 6 posters. Thanks to the NOAA
Modeling, Analysis, Predictions and Projections (MAPP)
Program, the workshop was live-streamed as a CLIVAR-NOAA
webinar.
This CLIVAR Exchanges Special Issue highlights some
of the topics presented and discussed at the workshop.
Most of the presentations and abstracts, as well as more
detailed information on the workshop, are available at:
http://www.clivar.org/omdp/kuroshio
Workshop motivation and objectives
The Kuroshio, the western boundary current of the winddriven gyre of the North Pacific Ocean, and the Kuroshio
Extension (KE) current form a complex system along with
other components in the western North Pacific. This system
impacts on the local and global climate system in various
ways. The Kuroshio transports a vast amount of heat from
low to mid-latitudes, affecting both the global-scale energy
balance and local climate. The KE and the recirculation gyre
formed to the south of the Kuroshio act as a heat reservoir,
penetrating through the atmospheric boundary layer as
far as the upper troposphere. There is a strong meridional
sea surface temperature (SST) gradient north of the KE,
contributing to atmospheric storm activity. These significant
impacts underline the importance of understanding the role of
the KE on the climate and its variability.
For climate modelling to reproduce this complicated system
we need ocean models with a horizontal resolution of at least
about 20 km (eddy permitting), ideally 10 km or less (eddy
resolving). Thanks to the enhancement of computational
resources such models are now commonly applied to many
related scientific topics. The “CLIVAR Working Group on Ocean
Model Development (predecessor of OMDP) Workshop on
High Resolution Ocean Climate Modelling” in 2014 highlighted
many interesting studies with ocean models of 1/4o resolution
or finer. Many climate modelling centres are planning to employ
such high resolution ocean models in their contributions to the
forthcoming Coupled Model Intercomparison Project Phase
6 (CMIP6). So it was most timely to share our experience
and knowledge of theoretical, observational, and modelling
studies, deepen our mutual understanding, and seek new
collaborations during this workshop.
The objectives of the workshop were:
1. To assess the state-of-science of the theory, observations,
and ocean climate modelling of the Kuroshio Current and
Extension systems in the North Pacific Ocean;
2. To assess the role of higher resolution to improve the
simulation of the Kuroshio system in ocean climate
models;
3. To explore opportunities for new collaborative and focused
studies on the Kuroshio Current and Extension system to
improve decadal climate prediction capabilities both for
the Asia-Pacific region and globally.
Keynote speakers
After short introductory remarks, the workshop began with
two keynote presentations. Bo Qiu presented the KE decadal
variability. A KE index (Qiu et al., 2014), calculated from
four dynamical quantities related to the KE variability and
representing stable/unstable phases of KE, was presented.
The high correlation between the KE index and Pacific
Decadal Oscillation (PDO) index shows that the large-scale
KE variability is driven by basin-wide wind forcing. In addition,
non-linear oceanic eddy forcing is found to enhance the lowfrequency variability in KE. This (decadal) KE variability
affects the overlying storm-track activity as well as the
regional sea level trends, mixed layer properties, and SST in
the western North Pacific. The atmospheric response in turn
enhances the KE decadal variations via oceanic feedbacks.
Ichiro Yasuda’s presentation focused on the impact of KE
variability on the marine ecosystem variability and climate.
Significant inter-decadal variability has been observed in
the catch of Japanese sardine, which lives in the KE region.
The winter SST and mixed layer depth changes along the
Kuroshio front have significant impacts on food availability for
sardine larvae and subsequent survival of Japanese sardine.
The Japanese sardine catch variability is coherent with
northwestern U.S. climate variability that can be explained
by the PDO’s bi-decadal and penta-decadal components. The
bi-decadal PDO component is closely related to the 18.6-yr
period tidal oscillation through strong tidal mixing along the
Kuril Straits. Surface and subsurface signals generated by this
strong mixing propagate to the KE region and interact with the
atmosphere.
Observation of the Kuroshio and KE
As one of the largest western boundary currents in the global
ocean, the Kuroshio Current is an important and biologically
productive system. Understanding the mechanisms of
its variability helps to elucidate impacts on the marine
ecosystem, ocean productivity, and climate change.
Meghan Cronin presented how data from the KE Observatory
(KEO) surface mooring, combined with satellite SST data,
are used to obtain the diffusivity values based on the mixed
layer heat balance at KEO. By evaluating the budgets for both
dissolved inorganic carbon and alkalinity from this estimated
diffusivity, the export rate of both organic and inorganic
carbon from the surface mixed layer can be distinguished and
quantified, providing a baseline for the biological carbon pump
in this dominant carbon sink region of the North Pacific Ocean.
Toshio Suga gave a review of the role of the KE in the
ventilation of the North Pacific pycnocline. The wintertime
mixed layers to both the south and north of the KE are among
the deepest mixed layers in the North Pacific. Part of the large
volume of water with properties renewed in this deep mixed
layer spreads widely and ventilates a substantial part of the
North Pacific pycnocline. The observed characteristics of
this region as a ventilator of the pycnocline were reviewed.
Variability of the upstream region of the Kuroshio was presented
by Kaoru Ichikawa. The variations of the position and speed of
the Kuroshio axis were described. It is found that the Kuroshio
current moves southward and its width becomes narrower
when the axis speed is higher, and vice versa. Variations are
revealed to be associated with anticyclonic (or cyclonic)
offshore mesoscale eddies approaching from the east.
The interannual to decadal variability of KE was examined in
Yoshinori Sasaki’s presentation. Sea level variability along the
Japanese coast is quite strong in the regions that are under
the direct influence of jet-trapped Rossby waves, implying that
the wind-driven circulation changes have significant impacts
on the sea level variability in the KE region and along the coast
of Japan.
Takeyoshi Nagai presented evidence of enhanced doublediffusive convection below the main stream of the KE.
This research indicates that double-diffusive convection is
enhanced by mesoscale subduction/obduction and nearCLIVAR Exchanges No. 69, Vol. 20, No. 1, July 2016
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inertial motions, and thereby diapycnal tracer fluxes could be
enhanced below the KE front.
Ocean Dynamics in the Kuroshio and KE,
including multi-scale ocean processes
This session addressed notable ocean processes related to
ocean and climate dynamics from various aspects: hot spots,
surface wave mixing, water mass and front formation, ocean
heat content variability, momentum balance of the KE, and
regional sea level changes.
Xiaopei Lin highlighted the hot spots of the Kuroshio and
KE. SST trends in the KE are affected by Kuroshio decadal
variability driven by higher-latitude large-scale wind patterns.
Multi-scale interaction is important to understand the Kuroshio
variability. On behalf of Fangli Qiao, he explained that the
mixing induced by non-breaking waves may add an important
contribution to the vertical mixing process in the upper ocean.
Using the climate model FIO-ESM with a parameterization
of such mixing, the volume transport of Kuroshio in the East
China Sea was realistically simulated.
Hideyuki Nakano addressed the formation of water masses
in the northwestern Pacific. The Kuroshio-origin water is
distributed in the subtropical gyre through Sverdrup circulation
simulated using an eddy-resolving model including virtual
tracers. There is a significant contribution from the marginal
seas in the water mass distribution to the east of Japan.
Naoki Sato discussed the seasonal characteristics of the KE
in early summer. Wind stress curl plays an important role in
maintaining the meridional SST gradient along the KE through
Ekman upwelling.
Bunmei Taguchi highlighted interannual to decadal ocean
heat content variability in the North Pacific, addressing
the generation and propagation processes of heat content
anomalies. The eastward propagation occurs through
anomalous spiciness advection and westward propagation
via heaving of the thermocline. The spiciness signals are
generated in the subarctic frontal zone near Japan.
Kunihiro Aoki discussed the Reynolds stress and interfacial
form stress in the momentum equations to explain the
downstream time mean decay of the KE jet. The eastward
deceleration of the KE jet is governed by the ageostrophic
Coriolis force and the meridional integral of the zonal Reynolds
stress. The ageostrophic flow is partially driven by eddy kinetic
energy (EKE). The relationship between the ageostrophic
flow and EKE in the temporal mean may be explained by the
mechanism of eddies accompanying curvature effect and
migrating over the KE region.
Mio Terada reported on regional sea level change over the
western North Pacific to the end of the 23rd century in
CMIP5 models. The spatial patterns of regional sea level
change evolve over the 21st, 22nd and 23rd centuries due
to the intensification and the northward shift of the KE.
Development of ocean modelling: highresolution modelling and reanalysis
This session explored impacts of bottom topography and
resolutions of ocean models. Exciting developments exploring
effects on climate and the Kuroshio of using high-resolution
ocean models at regional to global scales were highlighted.
Masao Kurogi showed the effect of deep trenches and related
bathymetry on the sea surface height field in the KE. Nested
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high-resolution models are used, with resolutions of as high as
400-m, included barotropic and baroclinic dynamics, tracer
advection, and sea-ice interactions. Simulations with a higher
resolution nested ocean model — 100m resolution — are
planned. Tsuyoshi Wakamatsu presented a 4D ocean data
assimilation study of Kuroshio variability. The 1982-2014
reanalysis at 1/10o horizontal resolution was forced by JRA55 reanalysis data. The Four-dimensional Variational Ocean
ReAnalysis for the Western North Pacific (FORA-WNP30)
product is available for research purposes at http://synthesis.
jamstec.go.jp/FORA/e/, and is a recommended dataset for
KE variability studies. It captures a series of stable/unstable
periods, and offers insights into subsurface variability.
Helene Hewitt spoke on impacts of resolving/permitting
mesoscale eddies in coupled climate simulations. A 1/12o global
ocean model (NEMO) has been coupled to an N512 resolution
atmosphere. Preliminary results indicate a 20% reduction in
the Southern Ocean warm bias, a reduction in cold biases in
the N. Hemisphere and some improvement in upwelling bias
regions. Tides and shelf processes are resolved to study
carbon update, marine impacts, and marine methane release.
Simona Masina explored KE decadal variability with a 1/4o
horizontal resolution, 50-level version of NEMO, showing a
large improvement from data assimilation over a free-running
simulation with stable and unstable KE states reproduced in
the former and not the latter. A new KE index that combines
integrated wavelet amplitude and mean KE latitude is
used to distinguish stable and unstable states of the KE.
Eric Chassignet presented a detailed study of the Gulf
Stream separation, utilizing simulations with horizontal
resolutions from 1/12o to 1/50o. The results were discussed
in terms of power spectra with comparisons to the Kuroshio.
Air-sea interaction in the Kuroshio and KE and
its climate impact
Presentations focused on air-sea heat fluxes, the atmospheric
boundary layer, the free troposphere, and basin-scale
atmospheric circulations.
Shoshiro Minobe introduced the newly formed CLIVAR
Climate Dynamics Panel that fosters increased understanding
of the dynamical processes that control circulation variability
and change in the atmosphere and ocean on synoptic to
centennial timescales. The focus of the Panel is on largescale phenomena, processes, and mechanisms of coupled
climate variability/modes, teleconnnections and change
on seasonal to centennial time-scales. At mid-latitudes, the
ocean’s influence on the atmosphere is especially strong along
western boundary currents, such as the Kuroshio and the Gulf
Stream, which provide heat and moisture to the atmosphere.
Studies of the mid-latitude ocean’s influence on the
atmosphere have rapidly developed in the last one and half
decade, exploiting high-resolution observational data and
numerical modelling. Atmospheric patterns coherent with
oceanic structures, such as fronts and currents can be
detected with high-resolution observational data analysis.
Such studies indicate that oceanic forcing play an important
role in the atmosphere given that scales of free-atmospheric
variations are one order larger than oceanic ones. Using a new
diagnostic for time-mean near-surface winds, Minobe showed
different contributions of pressure adjustment and vertical
mixing mechanisms; the former responsible for convergence/
divergence of near-surface winds, while the latter for their
rotation over the Gulf Stream, consistent with observational
analysis using atmospheric temperatures observed by AIRS.
This strategy is referred to as a “scale-separation strategy”.
An intrinsic limitation of this strategy exists when atmospheric
responses occur on atmospheric spatial scales when studies
must rely on numerical simulations.
Sergey Gulev presented air-sea heat fluxes associated with
cyclones and anticyclones, showing that extremely large
surface turbulent heat fluxes are associated with cycloneanticyclone interaction zones, and that about 30% of cyclones
propagating over the North Pacific provide more than 70% of
the total turbulent heat loss.
Niklas Schneider presented the results of a simple atmospheric
boundary layer model that resolves the two major mechanisms
of the atmospheric responses in the boundary layer, i.e., the
vertical mixing and pressure adjustment mechanisms. By
exploiting linear dynamics, spectral solutions are obtained that
are generally consistent with a high-resolution atmospheric
model.
Hisashi Nakamura presented a new product of the Japanese
Reanalysis (JRA-55HS) for which high-resolution SST is used
– in contrast to the standard JRA-55 product with lowerresolution SSTs. The JRA-55HS is available for the 1985-2012
period, much longer than the ERA-interim high-resolution
product. JRA-55HS is a legacy of the Japanese Hot-Spot project.
Using reanalysis data with high resolution SST, Ryusuke
Masunaga showed differences in heat fluxes north of the KE for
the stable and unstable regimes, and that high SST resolution
in reanalysis products (JRA-55HS and ERA-interim after
2002) is important for reproducing important SST-induced
atmospheric responses.
The importance of SST resolution is also shown in a numerical
study by Justin Small with two versions of the Community Earth
System Model (CESM): a high-resolution (nominal 0.1o) versus
a low-resolution (nominal 1.0o) ocean model coupled with the
same 0.25o atmospheric model. The study demonstrated
that interannual variability of ocean-to-atmosphere surface
turbulent heat fluxes are positively correlated with sea surface
height in high-resolution CESM in mid-latitude frontal regions
as observed, but this relationship is absent at low-resolution.
The local atmospheric response to SST fronts and eddies are
well established. However, possible remote and large-scale
atmospheric responses have much larger uncertainties. Ping
Chang showed that ocean mesoscale eddies exert substantial
impacts on the atmosphere even for basin-scale atmospheric
circulations. The large-scale atmospheric influence of ocean
eddies is a new finding, indicating the possible deficiency of
IPCC-class climate models that do not resolve ocean eddies.
Akira Kuwano-Yoshida conducted two 20-year experiments
with a 0.5o resolution atmospheric model; one with observed
0.25o SSTs and the other with smoothed SSTs in the
Kuroshio and Oyashio regions. Rapidly deepening cyclones
are more frequent in the control run than those in the
smoothed run. The difference in the cyclone genesis results
in basin-scale impacts on the atmospheric circulation and
precipitation along the western coast of North America.
Stuart Bishop analyzed 100-year integrations of CESM with
resolutions of 0.1o and 0.25o. The ocean and atmosphere
meridional heat fluxes compensate each other through
“Bjerknes Compenstation” in winter over the North Pacific.
Also, Shoshiro Minobe showed that the sharp SST front of the
Gulf Stream is important to reproduce a realistic European
blocking distribution and its associated cold-spells over Europe.
Several papers analyzed oceanic aspects of air-sea interactions.
Tomoi Tozuka examined the mechanism by which the SST
front of the KE is generated, using a heat budget in the mixed
layer. Although heat flux from the ocean to the atmosphere is
stronger south of the front, the weaker heat flux combined with
the shallower mixed layer to the north of the front cools SSTs
more efficiently there and thus produces a sharper SST front.
Masami Nonaka studied the interaction between the tropics
and mid-latitude North Pacific. Instead of the canonical El
Nino, the El Nino Modoki caused a lagged response over the
North Pacific, resulting in sea-surface height anomalies just
to the south of the KE 3.5-year later. The influence of El Nino
Modoki on the North Pacific is suggested to be enhanced in
the last two decades. Hidenori Aiki showed air-sea momentum
fluxes associated with oceanic surface waves in a regional airsea coupled model significantly impact SST fields associated
with tropical cyclones near Taiwan, suggesting that potential
feedback from the ocean to tropical cyclones may be
influenced with or without wave effect in the coupled model.
Marine ecosystems in the Kuroshio and KE
Hiroaki Saito presented an explanation of the “Kuroshio
paradox” — how can the nutrient depleted Kuroshio sustain
such diverse and abundant fish resources? Hapto-phyta
is the dominant phytoplankton in the Kuroshio, which
is different from other subtropical oligotrophic areas.
The nutrient supply occurs through various processes:
island and continental shelf sediments, riverine supply,
turbulent mixing, and upwelling via front genesis. The
food web is also different from nano-phytoplankton, here
being predominantly gelatinous to doliolids planktons.
Keith Rodgers presented results from simulations using
GFDL’s Earth system model ESM2M over the 1950-2100 period
in comparison with phenomena observed along the 165oE JMA
sections where decadal scale variability of subsurface pH and
oxygen are observed during 1987-2012. This presentation
pointed out that pH increasing trend at 26.6σθ is more obvious
than the trend of oxygen, suggesting that pH is more influenced
by anthropogenic acidification. On the other hand, the
subsurface oxygen variability is more influenced from natural
variability propagated from upstream Central Mode Water.
Eitarou Oka showed the impact on the western boundary
current regions from the formation of Subtropical Mode
Water (STMW) whose formation rate changes with the
decadal KE variability. After 2010, enhanced subduction of
STMW was related to increased dissolved oxygen, pH, and
aragonite saturation state. This coincided with decreased
potential vorticity, apparent oxygen utilization, nitrate,
and dissolved inorganic carbon. The changes of dissolved
inorganic carbon, pH, and aragonite saturation state were
opposite against their long-term trends. These results
indicate a new mechanism consisting of westward sea
surface height anomaly propagation, the KE state transition,
and the STMW formation and subduction, by which the
climate variability affects physical and biogeochemical
structures in the ocean’s interior and potentially impacts the
surface ocean acidification trend and biological production.
Workshop discussion and recommendations
There are several on-going and planned observational research
programs around the KE region. We discussed objectives,
research plans, and results of these programs: KEO surface
mooring by NOAA-PMEL; a Japanese KEO observing project
on the role of atmospheric dust in the marine biological pump;
MEXT KAKENHI Innovative Study Ocean Mixing Processes:
Impact on Biogeochemistry, Climate and Ecosystem (OMIX);
and IOC-AIKEC. Overlaps and gaps among the projects were
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examined, and the need and opportunities for future
international collaborations among these parallel observational
programs were vigorously discussed.
The KEO cruise will deploy 5000m deep sediment traps near
the KEO buoy to understand the missing source of nutrients
in ocean desert, in other words, the mechanisms of nutrient
supply in oligotrophic NW Sub-tropical Pacific. The study
will address how high primary production is supported or
maintained in this region, the sources of nutrients of the ocean
surface, the role of meso-scale eddies, meteorological events,
eolian input, N2 fixation and regeneration. Understanding
these processes require observations over multiple years, yet
funding for ship time must be applied for year by year. The
workshop participants were extremely enthusiastic of these
plans and sought to identify opportunities for collaboration.
To better understand the upper ocean carbon flux, the cruise
and sediment trap measurements could be combined with
glider deployment. Integrating the results with satellite data
would be very useful to interpret surface BGC and carbon flux.
The OMIX study will involve JMA annual CTC stations and
vertical mixing measurements over next 4-5 years. Two gliders
are planned to cross KE with microstructure measurements.
Meghan Cronin encouraged OMIX to undertake validation
measurements near the KEO buoy (CTD salinity measurements
and water samples if possible).
IOC-AIKEC cruises are being undertaken by China yearly
from 2014 to 2016. Moorings have been deployed near the
KEO buoy as well as CTD sections and 20 Argo floats. The
focus is on meso- and submeso-scale variability, inertial
oscillations and areas of intense air sea exchange. In terms
of long term plans, until 2020, an effort is underway to
enhance international observation collaboration in the W.
Pacific and E. Indian Ocean. 1 to 2 cruises are planned in
this period in KE region to deploy and recover moorings.
The cruises are open for participation and data exchange.
hand, numerical models can provide fields with fine spatial/
temporal resolution that cannot be obtained by observations.
Regarding this viewpoint, it should be noted that some
participants expressed their interest in JRA-55HS, a derivative
of the Japanese reanalysis product JRA-55, that uses a a highresolution SST dataset (Nakamura et al., this issue).
An interesting discussion followed on the challenges of
accessing observational data and the need for a “one-stop
shopping site”. Several participants noted that it is difficult
for many researchers to know where the observational data
are, and what data are included in the reanalysis datasets.
OceanSITES (http://www.oceansites.org/), part of the Global
Ocean Observing System, is in intended to be a one stop shop
for modellers to obtain documented, formatted data and
reference datasets for model development and testing. The
response to the question “Where’s the Data” is provided by
Meghan Cronin in this issue.
The need to constrain reanalysis products with observations
was emphasized. WMO reference datasets should not be
assimilated so they can be used to benchmark reanalysis
products. The recommendation is that modelling centres do
not assimilate reference data sets. If they are assimilated, it
should be clearly stated to avoid redundancy in the validation
of the reanalysis product. KEO sea level pressure has been
assimilated into JRA55. However, temperature and other
meteorological variables have not so these can be used to
validate the JRA55 product.
Overall, the participants agreed to enhancing inter- and intraobservational and modelling coordination and collaboration.
The workshop provided a great opportunity to share experience
and knowledge, and promote future communication and
collaboration among modelling/observational groups.
Results from the Japanese “Hot-Spot” extra-tropical air
sea interaction study, which concluded in March 2015, is
compiled in a special issue of J. Oceanography (October
2015). The NKEO buoy was deployed between KEO and
J-KEO and access to all observational data including buoy
data is open for the international community, following
JAMSTEC regulations. This data is useful for model
validation. Hope was expressed that Chinese studies will
continue to collect data to build on the Hot Spot legacy.
Meghan Cronin concluded this discussion describing KEO, a
component of the sustained observing system, as a magnet
for process studies and deployments enabling research on
processes, climate variability and change in the region. The
community should envisage extending these capabilities
to achieve a carbon ‘hot spot’ process study around KEO
deploying new technologies, e.g. the saildrone—an unmanned
sail boat, paired with underwater gliders. Observing
small scale variability in air sea fluxes can be achieved
by leveraging both Japanese and Chinese efforts, J-KEO
and C-KEO respectively, including buoys north of the KE.
The discussion extended to the collaboration and synergy
between observational and modelling studies. The Ocean
Model Intercomparison Project (OMIP), which OMDP oversees
as one of the endorsed MIPs in CMIP6, was introduced. Reliable
observational data, preferably of long duration, is extremely
important in order to evaluate model simulations. Reanalysis
data also inherently depends on observations. On the other
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Figure : A group photo, at the entrance hall of JAMSTEC Yokohama
Institute, January 13, 2016.
Acknowledgements
We, the scientific steering committee members, are grateful
for sponsorship from Climate and Ocean – Variability,
Predictability, and Change (CLIVAR), US CLIVAR, Japan
Agency for Marine-Earth Science and Technology (JAMSTEC),
and Intergovernmental Oceanographic Commission (IOC).
This workshop was also supported by MEXT Grant-in-Aid for
Scientific Research on Innovative Areas Number H05825. We
are also grateful to the NOAA Modeling, Analysis, Predictions
and Projections (MAPP) Program for their generous support
in live streaming the workshop. We would like to thank Prof.
Bo Qiu for organizing a discussion session in the workshop
and providing valuable comments for this article. Thanks are
extended to the local administrative staff, the oral and poster
presenters, and all the participants for their contributions that
made the workshop a great success.
Inter-Decadal
Modulations in the
Dynamical State
of the Kuroshio
Extension System:
1905-2015
References
Cronin, M. F., 2016: Where’s the Data? CLIVAR Exchanges 69
[this issue]
Nakamura H., Y. Kawai, R. Masunaga, H. Kamahori, C.
Kobayashi, and M. Koike, 2016: An extra product of the JRA55 atmospheric reanalysis and in-situ observations in the
Kuroshio-Oyashio Extension under the Japanese “Hotspot
project.” CLIVAR Exchanges 69 [this issue]
Qiu, B., S. Chen, N. Schneider, and B. Taguchi, 2014: A coupled
decadal prediction of the dynamic state of the Kuroshio
Extension system. J. Climate, 27, 1751-1764.
Bo Qiu, Shuiming Chen, and Niklas
Schneider
Department of Oceanography, University of Hawaii
at Manoa
Introduction
After separating from the Japanese coast at 36°N, 141°E, the
Kuroshio enters the open basin of the North Pacific, where
it is renamed the Kuroshio Extension (KE). Free from the
constraint of coastal boundaries, the KE has been observed
to be an eastward-flowing inertial jet accompanied by largeamplitude meanders and energetic pinched-off eddies (see,
e.g., Qiu, 2002a for a comprehensive review). Compared to its
upstream counterpart south of Japan, the KE is accompanied
by a stronger southern recirculation gyre (RG) that increases
the KE’s eastward volume transport to more than twice the
maximum Sverdrup transport (~60Sv) in the subtropical
North Pacific Ocean, enhancing its nonlinear nature as a
western boundary current extension. In addition to the high
level of mesoscale eddy variability, an important feature
emerging from recent satellite altimeter measurements and
eddy-resolving ocean model simulations, is that the KE system
exhibits clearly-defined decadal modulations between a stable
and an unstable dynamic state (e.g., Qiu, 2003; Qiu & Chen,
2005, 2010; Taguchi et al., 2007, 2010; Cebollas et al. 2009;
Sugimoto and Hanawa, 2009; Kelly et al. 2010; Sasaki et al.,
2013). As shown on the cover page of this Exchanges issue,
the KE paths were relatively stable in 1993–1995, 2002–
2005 and 2010-2015. In contrast, spatially convoluted paths
prevailed during 1996–2001 and 2006–2009. It is important
to emphasize that these changes in path stability are merely
one manifestation of the decadally-modulating KE system.
When the KE jet is in a stable dynamic state, available satellite
altimeter data further reveal that its eastward transport and
latitudinal position tend to be greater and more northerly,
its southern RG tends to strengthen, and the regional eddy
kinetic energy level tends to decrease. The reverse is true
when the KE jet switches to an unstable dynamic state.
KE index during the satellite era
To succinctly summarize the time-varying dynamical state
of the KE system, Qiu et al. (2014) introduced the KE index
defined as average of the variance-normalized time series of
6
the southern RG intensity, the KE’s intensity, its latitudinal
position, and negative of its path length. A positive KE
index, thus defined, indicates a stable dynamical state and
a negative KE index, an unstable dynamical state. The red
line in Figure 1c shows the KE index in the satellite altimetry
period of 1993–2015. Dominance of the decadal modulation
in the KE system is easily discernible in this time series.
Transitions between the KE’s two dynamical states are caused
by the basin-scale wind stress curl forcing in the eastern North
Pacific related to the Pacific decadal oscillations (PDOs).
Specifically, when the central North Pacific wind stress curl
anomalies are positive (i.e. positive PDO phase), enhanced
Ekman flux divergence generates negative local sea surface
height (SSH) anomalies in 170°–150°W along the southern RG
latitude (see Figures 1a and 1b). As these wind-induced negative
SSH anomalies propagate westward as baroclinic Rossby
waves into the KE region after a delay of 3–4 years, they weaken
the zonal KE jet, leading to an unstable (i.e., negative index)
state, of the KE system with a reduced recirculation gyre and
an active eddy kinetic energy field. The negative, anomalous
wind stress curl forcing during the negative PDO phase, on the
other hand, generates positive SSH anomalies through the
Ekman flux convergence in the eastern North Pacific. After
propagating into the KE region in the west, these anomalies
stabilize the KE system by increasing the KE transport and by
shifting its position northward, leading to a positive index state.
A regression analysis in Qiu et al. (2014) reveals that the KE
index defined by the 4 dynamical properties can be favourably
represented by the sea surface height (SSH) signals inside
the KE’s southern RG region of 31°–36°N and 140°–165°E
(black lines in Figure 1c). The linear correlation coefficient
between the red and thick (thin) black lines in Figure 1c is
as high as 0.97 (0.84). Physically, this high correlation is
of little surprise because the large-scale KE variability is
closely entwined to the dynamical state of its southern RG
and the RG’s variability is well represented by its regional
SSH signals. On a practical level, this makes exploration
of the KE dynamical state be equivalent to examination of
the SSH signals in the key region of the KE’s southern RG.
As evidenced in Figure 1b, changes in the mid-latitude windforced SSH field is governed by westward-propagating Rossby
wave adjustment processes (see Qiu, 2002b, and references
therein, for theoretical justification). Figure 1d shows the SSH
time series averaged in the 31°–36°N and 140°–165°E box
based on the 1.5-layer reduced-gravity model forced by the
monthly ECMWF ERA-Interim wind stress data (Dee et al.,
2014). The correlation between this modelled SSH time series
and that shown in Figure 1c (thick black line) reaches 0.85,
demonstrating the usefulness of the wind-forced 1.5-layer
reduced-gravity model in capturing the decadal modulations
of the KE dynamical state during the past quarter of a century.
KE index over the past century
Use of the SSH signals in the southern RG box as a proxy
for the KE dynamical state allows us to explore the KE index
variations beyond the satellite altimetry era. To achieve this,
we merge the ECMWF reanalysis wind stress product ERA20C (available from 1900 to 2010; Stickler et al., 2014) and
ERA-Interim (available from 1979 to 2015) and force the
1.5-layer reduced-gravity model (over the overlapping period
of 1979-2010, the merge is done bi-linearly). Figure 2a shows
the model-derived SSH changes in the KE’s southern RG box of
31°–36°N and 140°–165°E. Notice that to focus on the decadal
variability, we have removed the long-term increasing linear
trend of 1.5 cm/decade from Figure 2a; also, no SSH signals
are evaluated in the first 5 years due to the model spin-up. For
comparison and serving as an independent check, we plot in
Figure 2b the time series of surface dynamic height calculated
from the historical, objectively-analyzed, T/S data compiled
by Ishii et al. (2006) in the same southern RG box from 1945 to
2012. Because the available T/S data is confined to the upper
ocean of 1,500m, the amplitude of the T/S-based KE index
is, on average, about half that derived from the wind-forced
1.5-layer reduced-gravity model. However, the observed
phase changes of the KE index agree well with those predicted
by the wind-forced model hindcast. Note that the agreement
between Figures 2a and 2b improves after 1960s when the
in-situ T/S measurements became more abundant: the linear
correlation coefficient between the two time series in 1960–
2012 is 0.82, as compared to 0.74 during the overlapping
period of 1945–2012.
Figure 1: (a) Time series of Pacific decadal oscillation (PDO) from
http://jisao.washington.edu/pdo/PDO.latest. (b) SSH anomalies along
the zonal band of 32°-34°N from the AVISO satellite altimeter data. (c)
Time series of the KE index synthesized from four dynamical properties
(red line) versus the normalized SSH anomalies averaged in 31°-36°N
and 140°-165°E (black lines); here, the thin black line denotes the weekly
time series and the thick black line the low-pass filtered time series. (d)
SSH anomalies in the 31°-36°N and 140°-165°E box predicted by the
1.5-layer reduced-gravity model forced by the ECMWF Interim wind
stress data.
7
Both Figures 2a and 2b reveal that the low-frequency
modulation of the KE dynamical state is not confined to the
last two decades during which we had satellite-based SSH
information to capture the detailed evolution of the KE system.
To examine how the dominant period of the century-long KE
index has modulated over the past century, we plot in Figure
2c the wavelet power spectrum for the KE index time series
shown in Figure 2a. Large-amplitude decadal changes can be
seen to persist after the mid-1970s. From mid-1940s to mid1970s, the KE index appears to have two dominant periods:
one in the 15~20-year band and the other in the 4~6-year
band. Note that the short 4~6-year variability in the KE index
was also detected in the eddy-resolving OFES model output
of 1955–1975 that was forced by the NCEP-NCAR reanalysis
wind stress data (Qiu et al., 2014; their Figure 5a). In between
the mid-1920s and mid-1940s, the KE index appears to be
dominated by the 10~15-year fluctuations and prior to the
mid-1920s, Figure 2c indicates that the predominant period
KE state tends to generate a positive (negative) wind stress
curl in the eastern North Pacific basin, resulting in negative
(positive) local SSH anomalies through Ekman divergence
(convergence). This impact on wind stress induces a delayed
negative feedback with a preferred period of ~10 years
and is likely the cause for the enhanced decadal variance
demonstrated in Figure 2. It will be important for future studies
to clarify if similar feedback mechanisms are at work within
the epochs of other climatic regime shifts in the 20th century.
Acknowledgments
The ERA-Interim and ERA-20C surface wind stress
data are provided by ECMWF and the merged satellite
altimeter data by the Copernicus Marine and Environment
Monitoring Service (CMEMS). Support from NASA
NNX13AE51G and NSF OCE-0926594 is acknowledged.
References
Ceballos, L., E. Di Lorenzo, C. D. Hoyos, N. Schneider, and B.
Taguchi, 2009: North Pacific Gyre oscillation synchronizes
climate variability in the eastern and western boundary current
systems. J. Climate, 22, 5163-5174.
Dee, D. P., M. Balmaseda, G. Balsamo, R. Engelen, A. J.
Simmons, and J.-N. Thépaut, 2014: Toward a Consistent
Reanalysis of the Climate System. Bull. Amer. Meteor. Soc.,
95, 1235-1248.
Frankignoul, C., and N. Sennéchael, Y.-O. Kwon, and M.A.
Alexander, 2011: Influence of the meridional shifts of the
Kuroshio and the Oyashio Extensions on the atmospheric
circulation. J. Climate, 24, 762-777.
Kelly, K.A., R.J. Small, R.M. Samelson, B. Qiu, T.M. Joyce, Y.O. Kwon and M.F. Cronin, 2010: Western boundary currents
and frontal air-sea interaction: Gulf Stream and Kuroshio
Extension. J. Climate, 23, 5644-5667.
Figure 2: (a) KE index time series based on the SSH anomalies in the
31°-36°N and 140°-165°E box predicted by the 1.5-layer reduced-gravity
model forced by the merged ERA-20C and Interim wind stress data.
(b) KE index time series based on the SSH anomalies in the 31°-36°N
and 140°-165°E box calculated from the historical T/S data of Ishii et al.
(2006). Notice the difference from (a) in y-axis scale. (c) Wavelet power
spectrum for the time series of (a).
of the time-varying KE index falls in between 6 and 10 years.
It is worth emphasizing that the mid-1920s, mid-1940s, and
mid-1970s marked the three 20th-century climatic regime
shifts in the Aleutian Low pressure system over the North
Pacific Ocean (e.g., Minobe, 1997; Zhang et al., 1997). The
results in Figure 2 clearly indicate that these regime shifts in
the atmospheric forcing field exert a significant impact upon
the frequency content of the time-varying KE system. What is
yet to be explored and quantified is the degree to which the
variability in the KE dynamical state may feedback to the lowfrequency changes in the overlying atmospheric circulation.
By transporting warmer tropical water to the mid-latitude
ocean, the expansive KE provides a significant source of heat
and moisture for the North Pacific mid-latitude atmospheric
storm tracks (e.g., Nakamura et al., 2004). By modifying the
path and intensity of the wintertime overlying storm tracks,
changes in the KE dynamic state can alter not only the stability
and pressure gradient within the local atmospheric boundary
layer, but also the basin-scale wind stress pattern (Taguchi et
al., 2009; Kwon et al., 2010; Frankignoul et al., 2011; Kwon and
Joyce, 2013). After the latest regime shift of the mid-1970s,
Qiu et al. (2007; 2014) show that a positive (negative) index
Kwon, Y.-O., and T.M. Joyce, 2013: Northern hemisphere
winter atmospheric transient eddy heat fluxes and the Gulf
Stream and Kuroshio-Oyashio Extension variability. J. Climate,
26, 9839-9859.
Kwon, Y.-O., M. A. Alenxader, N. A. Bond, C. Frankignoul,
H. Nakamura, B. Qiu, and L. Thompson, 2010: Role of the
Gulf Stream and Kuroshio-Oyashio Systems in large-scale
atmosphere-ocean interaction: A review. J. Climate, 23, 32493281.
Ishii, M., M. Kimoto, K. Sakamoto, and S.-I. Iwasaki, 2006:
Steric sea level changes estimated from historical ocean
subsurface temperature and salinity analyses. J. Oceanogr.,
62, 155-170.
Minobe, S., 1997: A 50–70 year climatic oscillation over the
North Pacific and North America. Geophys. Res. Lett., 24, 683686.
Nakamura, H., T. Sampe, Y. Tanimoto, and A. Shimpo, 2004:
Observed associations among storm tracks, jet streams
and midlatitude oceanic fronts. Earth’s Climate: The OceanAtmosphere Interaction, Geophys. Monogr., 147, Amer.
Geophys. Union, 329-346.
Qiu, B., 2002a: The Kuroshio Extension system: Its largescale variability and role in the midlatitude ocean-atmosphere
interaction. J. Oceanogr., 58, 57-75.
Qiu, B., 2002b: Large-scale variability in the midlatitude
8
subtropical and subpolar North Pacific Ocean: Observations
and causes. J. Phys. Oceanogr., 32, 353-375.
Qiu, B., 2003: Kuroshio Extension variability and forcing of
the Pacific decadal oscillations: Responses and potential
feedback. J. Phys. Oceanogr., 33, 2465-2482.
Qiu, B., and S. Chen, 2005: Variability of the Kuroshio Extension
jet, recirculation gyre and mesoscale eddies on decadal
timescales. J. Phys. Oceanogr., 35, 2090-2103.
Qiu, B., and S. Chen, 2010: Eddy-mean flow interaction in the
decadally modulating Kuroshio Extension system. Deep-Sea
Res. II, 57, 1098-1110.
Variability and
Mixing in the
Kuroshio and Impact
on Ecosystem and
Climate
Qiu, B., N. Schneider, and S. Chen, 2007: Coupled decadal
variability in the North Pacific: An observationally-constrained
idealized model. J. Climate, 20, 3602-3620.
Ichiro Yasuda
Extension system. J. Climate, 27, 1751-1764.
Atmosphere and Ocean Research Institute, The
University of Tokyo, Japan
Sasaki, Y. N, S. Minobe and N. Schneider, 2013: Decadal
response of the Kuroshio Extension jet to Rossby waves:
Observation and thin-jet theory. J. Phys. Oceanogr., 43, 442456.
Introduction
Stickler, A., S. Brönnimann, M. A. Valente, J. Bethke, A. Sterin,
S. Jourdain, E. Roucaute, M. V. Vasquez, D. A. Reyes, R. Allan,
and D. Dee, 2014: ERA-CLIM: Historical Surface and Upper-Air
Data for Future Reanalyses. Bull. Amer. Meteor. Soc., 95, 14191430.
Sugimoto, S., and K. Hanawa, 2009: Decadal and interdecadal
variations of the Aleutian Low activity and their relation to
upper oceanic variations over the North Pacific. J. Meteor.
Soc. Japan, 87, 601-614.
Taguchi, B., H. Nakamura, M. Nonaka, and S.P. Xie, 2009:
Influences of the Kuroshio/Oyashio Extensions on air-sea heat
exchanges and storm-track activity as revealed in regional
atmospheric model simulations for the 2003/04 cold season.
J. Climate, 22, 6536-6560.
Taguchi, B., S.-P. Xie, N. Schneider, M. Nonaka, H. Sasaki, and
Y. Sasai, 2007: Decadal variability of the Kuroshio Extension.
Observations and an eddy-resolving model hindcast. J.
Climate, 20, 2357-2377.
Taguchi, B., B. Qiu, M. Nonaka, H. Sasaki, S.-P. Xie, and N.
Schneider, 2010: Decadal variability of the Kuroshio Extension:
mesoscale eddies and recirculations. Ocean Dyn., 60, 673691.
Zhang, Y., J. M. Wallace, and D. S. Battisti, 1997: ENSO-like
Interdecadal Variability: 1900–93. J. Climate, 10, 1004-1020.
9
Decadal to inter-decadal variability of the Kuroshio and the
Kuroshio Extension has a tremendous impact on ecosystem and
fisheries around Japan. One example is the Japanese sardine
(Sardinops melanostictus) which showed drastic variability in
the last three centuries (e.g. Yasuda et al., 1999). The main
causes of the Kuroshio Current system variability and thus the
Japanese sardine have been attributed to the basin-scale North
Pacific variability such as Pacific Decadal Oscillation (PDO,
Mantua et al., 1997). PDO has two dominant inter-decadal
components, bi-decadal and penta-decadal variability; the
latter penta-decadal period is 3 times the bi-decadal one, and
these two components change signs nearly simultaneously to
cause remarkable regime shifts (Minobe, 1999; Minobe and
Jin, 2004). Although reasons that determines the timescale
of the bi-decadal and penta-decadal period variability are not
been yet known, one possibility is the 18.6-year lunar tidal
cycle and the resulting vertical mixing variability in the ocean
(e.g. Loder and Garrett, 1978), which could connect all the
variability. This report reviews the recent studies on sardine
variability and 18.6-year period tidal cycle, and introduces a
Japanese 5-year project “Ocean Mixing Processes: Impact
on Biogeochemistry, Climate and Ecosystem (OMIX)”.
Japanese sardine variability
In the 20th century, catch of the Japanese sardine had two
peaks in the 1930s and 1980s. Sardine catches started to
increase in the early 1970s, peaked in 1988 (it reached over
4.5 million tons which corresponds to over 1/3 of Japanese
total catch and 1/20 of the world total), and has declined
since 1989. The decline is credited to the continuous very low
survival rates of 1988-1991 year classes and subsequent low
rates in the 1990s (Watanabe et al., 1995 and subsequent data
from Japan Fisheries Research Agency). Year-to-year survival
rate variability during 1979-1994 was found to be negatively
correlated with January-April sea surface temperature (SST)
(Noto and Yasuda, 1999; 2003) and winter mixed layer depth
(MLD) (Nishikawa and Yasuda, 2008) in the area south of
the Kuroshio Extension (30-35oN, 145o-180oE) whose winterspring warm (cold) and shallow (deep) winter mixed layer well
corresponds to the low (high) survival rates during 1988-1991
(1979-1987). Further detailed studies using high resolution
hindcast modelling showed that SST and MLD variability south
of the Kuroshio Extension correspond well to winter SST and
MLD variability along the Kuroshio main stream (Nishikawa
and Yasuda, 2011) where sardine larvae are transported to
the Kuroshio Extension (Nishikawa et al., 2011; 2013a; Itoh
et al., 2009; 2011), especially along the northern side of the
stream axis. In this region winter deep ML and winter-spring
low SST could cause high survival through nutrient supply
during winter and subsequent enhanced spring plankton
bloom and appropriate temperature environment (Nishikawa
et al., 2013a; 2013b). In the frontal zone on the northern side
of the Kuroshio axis, vertical mixing and nitrate diffusive flux
are significantly enhanced (Kaneko et al., 2012; 2013), and
could maintain the high biological production zone just north
of the Kuroshio Extension Front even after spring bloom,
when sardine larvae are transported and enhanced vertical
mixing increases the productivity (Nishikawa et al., 2013b).
The decadal winter SST and MLD variability are caused by
variations in the Kuroshio current speed (Fig. 1b) due to: i)
the arrival of westward propagating baroclinic Rossby waves
excited by Aleutian Low variability, and ii) atmospheric cooling
variability; The relative contribution of i) and ii) is 2:1 to SST/
MLD variability (Nishikawa and Yasuda, 2011).
Figure 1: (a) Schematic representation showing the relationship
between Japanese sardine and the Kuroshio decadal-scale variability
(modified from Fig. 12 of Nishikawa et al., 2013b). Intensification
(weakening) of the Kuroshio velocity (b) (modified from Fig. 14b of
Nishikawa and Yasuda 2011) as well as weakened (intensified) winter
cooling (c) correspond to warm (cold) and shallow (deep) winter
mixed layer denoted by red (blue) horizontal bars (modified from Fig.
12b of Nishikawa and Yasuda 2011). Sardines spawned south of Japan
in winter are transported along the Kuroshio where sardine larvae are
better survived in the environment of colder and deeper mixed layer
with higher nutrients which lead to greater and longer-lasting spring
bloom because of the enhanced vertical mixing along the northern side
of the Kuroshio.
PDO and 18.6-year lunar cycle
PDO consists of co-variability of mid-latitude SST and sea level
pressure (SLP) of Aleutian Low Pressure system. The SST and
SLP difference between the strong and weak diurnal tides in
the 18.6-year cycle is similar to the negative PDO pattern (Fig.
2a, modified from Fig. 1 of Yasuda et al. 2006); the Aleutian
Low pressure is weak and positive SST anomalies (SSTa) east
of Japan and in the central North Pacific and negative SSTa
along the west coast of North America. The 18.6-year period
lunar tidal cycle is caused by the variation of inclination of moon
orbital plane to the earth equatorial plane from 18.4o to 28.4o.
In the former, semi-diurnal (diurnal) tide is strong (weak), and
the opposite is true in the latter. The amplitude modulation
rates of diurnal (semi-diurnal) tidal forces change by 19% in O1
and 11% in K1 (4% in M2 and 30% in K2) to each mean amplitude.
The diurnal tides in the Kuril Straits are quite strong due to
the resonance between the diurnal tidal forcing and diurnal
topographically trapped internal tidal waves around Kuril
Islands. The very cold (2oC) subsurface water upwells due to
strong tide-induced turbulent vertical mixing as shown in Fig.
2c (Itoh et al., 2010; 2011; 2014; Yagi and Yasuda, 2012; 2013;
Ono et al., 2013; Yagi et al., 2014; Tanaka et al., 2014). This
enhanced turbulence changes by up to 20% in the 18.6-year
tidal cycle, and was hypothesized to lead to the bi-decadal
ocean and climate variability (Yasuda et al., 2006). In fact, in
the downstream regions of the enhanced diurnal tides through
the Kuril Straits and Aleutian Passes, surface to intermediate
depths bi-decadal variability have been demonstrated
on several variables: surface salinity, intermediate-depth
isopycnal oxygen, temperature, salinity and thickness in
the Oyashio and the Okhotsk Sea (Osafune and Yasuda,
2006) and eastern Bering Sea (Osafune and Yasuda, 2010;
2012), and phosphate in the Oyashio and east of Japan
(Tadokoro et al., 2009). Numerical modelling results of this
variability were shown by Osafune and Yasuda (2012; 2013).
The 18.6-year cycle is also seen in the climate indices such
as winter North Pacific Index (Aleutian Low Pressure),
winter East-Asian Monsoon Index and PDO (Yasuda et al.,
2006). 18.6-year spectrum peak is significantly detected
in the 270 year-long record of PDO reconstructed from
tree rings (Fig. 2b), and the phase is found to be locked to
the 18.6-year cycle (negative (positive)-PDO in the 3rd to
5th (9th to 12th) year from maximum diurnal tide (Yasuda,
2009). A climate model experiment with enhanced vertical
mixing around the Kuril Straits suggested that the impact
propagates as coastal and equatorial waves and affects the
ENSO and PDO (Hasumi et al., 2008). Another climate model
experiment with parameterized internal tidal mixing and 18.6yr variation in the global oceans yielded the similar observed
co-variability of SST and SLP of PDO and suggested the
importance of Kuril tidal mixing variability, which is amplified
by air-sea interaction between SST anomalies in the KuroshioOyashio Extension regions and the Aleutian Low (Tanaka
et al., 2012). Although these climate models reproduced
some key observational features, important features such
as time-lag, baroclinic waves and tropical/equatorial bidecadal variability have not been sufficiently reproduced,
probably because of unknown vertical mixing processes and
distributions and of insufficient mixing parameterization.
Future studies and OMIX
As stated above, many issues come from vertical mixing, which
is the most under sampled physical property in the oceans and
changes by order of 4 in space and time. This large variability
and its 18.6-year modulation could have tremendous impacts
on ocean circulation, biogeochemistry, climate and ocean
ecosystem/fisheries.
10
2015, to test hypotheses and resolve issues mentioned above,
by performing integrated observations and by developing
numerical models with vertical mixing processes. OMIX
welcomes international collaborators, with more information
available on the project website: http://omix.aori.u-tokyo.
ac.jp/en/.
References
Hasumi, H., I. Yasuda and H. Tatebe, M. Kimoto, 2008:
Pacific bidecadal variability regulated by tidal mixing
around the Kuril Islands. Geophys. Res. Lett., 35,
L14601,doi:10.1029/2008GL034406.
Itoh, S., I. Yasuda, H. Nishikawa, H. Sasaki and Y. Sasai, 2009:
Transport and environmental temperature variability of eggs
and larvae of the Japanese anchovy (Engraulis japonicus) and
Japanese sardine (Sardinops melanostictus) in the western
North Pacific estimated via numerical particle tracking
experiments. Fish. Oceanogr., 18(2), 118-133.
Itoh, S., I. Yasuda, T. Nakatsuka J. Nishioka, and Y. N. Volkov,
2010: Fine- and microstructure observations in the Urup Strait,
Kuril Islands, during August of 2006. J. Geophys. Res. Oceans,
115, C08004, doi:10.1029/2009JC005629
Itoh, S., T. Saruwatari, H. Nishikawa, I. Yasuda, K. Komatsu,
A. Tsuda, T. Setou, and M. Shimizu, 2011: Environmental
variability and growth histories of larval Japanese sardine
(Sardinops melanostictus) and Japanese anchovy (Engraulis
japonicus) near the frontal area of the Kuroshio. Fish.
Oceanogr., 20(2), 114-124.
Itoh, S., I. Yasuda, M. Yagi, H. Kaneko, S. Osafune, T. Nakatsuka
J. Nishioka, and Y. N. Volkov, 2011: Strong vertical mixing in
the Urup Strait, Geophys. Res. Letters, VOL. 38, L16607,
doi:10.1029/2011GL048507.
Figure 2: (a) Difference of SST (color shade in oC), SLP (contours in
hPa) and wind direction (arrows) between the periods of strong and
weak diurnal tide in the 18.6-year period tidal cycle (modified from
Fig. 1 of Yasuda et al., 2006). (b) Time-series of 5-year running mean
Pacific Decadal Oscillation (PDO) index (black) reconstructed from
tree-rings, 15.5-23.3-year bandpass (red) and inverted-18.6-year tidal
cycle (blue) (modified from Fig. 2 of Yasuda, 2009). (c) Satellite SST
on July 28 in 2006 showing the cold SST around the Kuril Straits due
to strong diurnal tidal mixing where the dark blue color denoting about
2oC. Orange color denotes about 20oC. Observations and climate model
experiments suggest that 18.6-year mixing variability around the Kuril
Straits regulates and yields the rhythm for the bi-decadal ocean and
climate variability such as PDO.
Vertical mixing could modify the deep and intermediate ocean
circulation. Enhanced vertical mixing in the western Pacific and
east-Asian marginal seas with relatively large internal tides,
strong western boundary currents and rough topography could
cause enhanced nutrient supply and biological production.
These could sustain the world largest biological uptake of
carbon dioxide (Takahashi et al., 2002) and abundant fisheries
with 26% landings in only 6% sea surface area. The enhanced
vertical mixing upwells iron to the surface, and could control
the biological production in the western subarctic high nutrient,
low chlorophyll (HNLC) areas (Nishioka et al., 2013) and in the
Green Belt along the continental slope of the eastern Bering
Sea (Tanaka et al., 2012; 2013; 2015). Bi-decadal and its 3
times long penta-decadal ocean and climate variability could
be explained by 18.6-year tidal cycle and its related variability.
Over 60 researchers launched a 5-year project OMIX in July
11
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climate oscillations over the North Pacific: Role in climatic
regime shifts, Geophys. Res. Lett., 26, 855– 858.
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and interdecadal climate oscillations through nonlinear
subharmonic resonance in delayed oscillators, Geophys. Res.
Lett., 31, L16206, doi:10.1029/2004GL019776.
Nishikawa, H. and I. Yasuda, 2008: Japanese sardine
(Sardinops melanostictus) mortality in relation to the winter
mixed layer depth in the Kuroshio Extension region. Fish.
Oceanogr. 17, 411–420.
Nishikawa, H. and I. Yasuda, 2011: Long-term variability of
winter mixed layer depth and temperature along the Kuroshio
jet in a high-resolution ocean general circulation model. J.
Oceanogr. 67, 503–518.
Nishikawa, H., I. Yasuda, and S. Itoh, 2011: Impact of winter-tospring environmental variability along the Kuroshio jet on the
recruitment of Japanese sardine (Sardinops melanostictus).
Fish. Oceanogr., 20(6), 570-582.
Nishikawa, H., I. Yasuda, S. Itoh, K. Komatsu, H. Sasaki, Y.
Sasai and Y. Oozeki, 2013a: Transport and survival of Japanese
sardine (Sardinops melanostictus) eggs and larvae via particle
tracking experiments. Fish. Oceanogr.,22(6), 509-522.
Nishikawa, H., I. Yasuda, K. Komatsu, H. Sasaki, Y. Sasai, T.
Setou and M. Shimizu, 2013b: Winter mixed layer depth and
spring bloom along the Kuroshio front: implications for feeding
environment and recruitment of Japanese sardine, Mar. Ecol.
Preg. Ser., 487, 217-229.
Nishioka, J., T. Nakatsuka, Y.W. Watanabe, I. Yasuda, K. Kuma,
H. Ogawa, N. Ebuchi, A. Scherbinin, Yu. N. Volkov, T. Shiraiwa,
M. Wakatsuchi, 2013: Intensive mixing along an Island chain
controls oceanic biogeochemical cycles. Global Biogeochem.
Cycles, 27, 1-10, doi:10.1002/gbc.20088.
Noto, M. and I. Yasuda, 1999: Population decline of the
Japanese sardine, Sardinops melanostictus, in relation to sea
surface temperature in the Kuroshio Extension. Can. J. Fish.
Aquat. Sci. 56, 973–983.
Noto, M. and I. Yasuda, 2003: Empirical biomass model for
the Japanese sardine with sea surface temperature in the
Kuroshio Extension. Fish. Oceanogr. 12, 1–9.
Ono, K., K-I Ohshima, T. Kono, K. Katsumata, I. Yasuda, and
M. Wakatsuchi, 2013: Distribution of vertical diffusivity in the
Bussol’ Strait: a mixing hot spot in the North Pacific. Deep-Sea
Res. I, 79, 62-73.
Osafune, S. and I. Yasuda, 2006: Bidecadal variability in the
intermediate waters of the northwestern subarctic Pacific and
the Okhotsk Sea in relation to 18.6-year perioid nodal tidal cycle.
J. Geophys. Res., 111, C05007, doi:10.1029/2005JC003277.
Osafune, S. and I. Yasuda, 2010: Bidecadal variability in the
Bering Sea and the relation with 18.6year perioid nodal tidal
cycle. J. Geophys. Res., 115, DOI: 10.1029/2008JC005110.
Osafune, S. and I. Yasuda, 2012: Numerical study on the impact
of the 18.6-year period nodal tidal cycle on water-masses in
the subarctic North Pacific. J. Geophys. Res.-Oceans, 117,
C05009 DOI: 10.1029/2011JC007734.
Osafune, S. and I. Yasuda, 2013: Remote impacts of the 18.6year period modulation of localized tidal mixing in the North
Pacific. J. Geophys. Res. Oceans. 118, 3128-3137.
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climatological surface ocean pCO2 and seasonal biological and
temperature effects. Deep-Sea Research II, 49, 1601–1622.
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southeastern Bering Sea. Geophys. Res. Lett., 39, L08603
DOI: 10.1029/2012GL051164.
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Numerical study on tidal mixing along the shelf break in the
Green Belt in the southeastern Bering Sea, J. Geophys. Res.
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12
Observations of the
Kuroshio Extension
by an Autonomous
Microstructure Float
Takeyoshi Nagai1, Ryuichiro Inoue2,
Amit Tandon3, Hidekatsu Yamazaki1
1. Department of Ocean Sciences, University of
Marine Science and Technology, Tokyo, Japan
2. Ocean Circulation Research Group, Japan
Agency for Marine-Earth Science and
Technology, Yokosuka City, Japan
3. Department of Mechanical Engineering,
University of Massachusetts Dartmouth, North
Dartmouth, Massachusetts, USA
Introduction
The Kuroshio carries tremendous volume of water and
transports large amount of heat, organic and inorganic
constituents, impacting climate, storm tracks, and ocean
ecosystems. The strong current of the Kuroshio is inherently
associated with its frontal structures, with across frontal
variations in temperature and salinity. Mixing of these water
properties could lead to formations of new watermasses, which
could be subducted to ocean interior at fronts (Rudnick 1996;
Nurser and Zhang 2000). Several studies have suggested
that the Gulf Stream, a western boundary current in the North
Atlantic, is a nutrient stream which transports nutrients from
tropical ocean to subpolar regions in its subsurface layers
(Pelegri and Csanady 1991; Pelegri et al. 1996). Recent studies
have shown that the subsurface current of the Kuroshio is also
a nutrient stream in the North Pacific (Guo et al. 2012 and 2013)
similarly to the Gulf Stream. Stirring and mixing along the path
of the Kuroshio, from the Kuroshio origin, its main stream off
Japan coast, and the Kuroshio Extension, could, therefore,
play important roles not only for physical oceanography and
air-sea interactions but also for biogeochemistry.
Subinertial stirring due to mesoscale flows moves water
parcels nearly along isopycnal, and, therefore, is not effective
to stir density of the water across the front. However, when
isopleths of temperature, salinity, and other tracers, for
example nutrients, are not parallel to isopycnals, it is very
efficient to stir these tracers resulting in numerous bands
of streaks or filaments, with no density signatures, so called
T-S compensated filaments (Rudnick and Ferrari 1999; Smith
and Ferrari 2009). This is the case in the Kuroshio Extension
Front, where the water parcels of same density consist of
different temperature and salinity across the front. With strong
baroclinicity and confluence in the Kuroshio, T-S compensated
anomalous features can be subducted on the dense side and
obducted on the light side of a front (Hoskins and Bretherton
1972). These T-S compensated filaments have been frequently
observed as tongues across the Kuroshio Extension Front in
13
the high resolution surveys (Nagai et al. 2012). The subinertial
stirring and associated filamentations increase tracer variance
with reductions of filaments’ spatial scales, cascading tracer
variance to smaller scales until it dissipates at the molecular
scale.
For the atmospheric fronts with the simultaneous deformation
by horizontal confluence and vertical shear, Haynes and
Anglade (1997) showed that the vertical diffusion process
is the vital dissipater of the tracer variance, which, in turn,
limits the horizontal scale of filaments. Using data obtained
from the North Atlantic Tracer Release Experiment (NATRE;
Ledwell et al. 1998), Ferrari and Polzin (2005) showed the
average balance between thermal variance provided by
mesoscale lateral stirring and average thermal dissipation
below 800 m depth, where intermediate and Mediterranean
waters convolve. Smith and Ferrari (2009) have also shown
numerically that vertical mixing is important and sufficient
to dissipate the tracer variance at these fine vertical scales,
consistent with the atmospheric study of Haynes and Anglade
(1997).
These previous atmospheric and oceanic studies suggest that
the vertical diffusion of the vertical filaments plays a crucial
role in determining the horizontal scale of the filaments.
However, it has been unclear what vertical mixing mechanisms
are responsible for dissipation of tracer variance.
Recent in situ observations in the thermocline under the main
stream of the Kuroshio show large turbulent kinetic energy
dissipation rates (Nagai et al. 2009), which are accompanied
by near-inertial internal wave shear along isopycnals (Nagai
et al. 2015a). Because near-inertial internal waves can
be trapped and amplified by geostrophic shears (Kunze
1985; Mooers 1975; Whitt and Thomas 2013), enhanced
turbulent dissipation with banded shear can be attributed
to turbulence associated with these near-inertial waves
under the Kuroshio. Even with no turbulence, tracers in the
ocean can be mixed vertically by double-diffusive convection
(Schmitt 1981; Schmitt and Georgi 1982; St. Laurent and
Schmitt 1999; Merryfield 2005; Schmitt et al. 2005). Because
T-S compensated filaments have no density signatures, it
is very plausible that these filaments are in double-diffusive
favourable conditions once they are formed vertically.
To observe these microscale mixing processes, tethered
vertical microstructure profilers have been utilized. However,
it is challenging to deploy such instruments within the strong
baroclinic currents of the Kuroshio, because quasi freefall or
free rising deployments, required for microstructure vertical
profiling, are easily prevented by strong currents, their vertical
shear, and varying wind conditions during the deployments.
Untethered instruments, such as floats and gliders are more
suitable to profile microstructure continuously under the
Kuroshio main stream with their slow profiling speeds, O(0.1
ms-1).
In this study, a Navis-MicroRider microstructure float and an
EM-APEX float were deployed along the Kuroshio Extension
Front. The results of the observations indicate that vertical
shear of near-inertial internal waves as well as subinertial flow
can create interleaving thermohaline structures along the
main stream of the Kuroshio Front, accompanied by very large
microscale thermal dissipation rates with weak turbulence,
suggesting that subinertial stirring and near-inertial shear
promote double-diffusive convection and forward cascade of
tracer variance.
Observations
Thermohaline structures and Microstructures
In July 2013, an EM-APEX float (Teledyne Webb Research;
Sanford et al. 2011) and a microstructure float were deployed
at 37oN 20’ 142oE 42’, which were advected rapidly along
the main stream of the Kuroshio Extension Front between
36oN and 37oN 30’ (Figure 2g). A microstructure sensor
package (MicroRider Rockland Scientific) and a battery
pack were mounted externally on a Navis-Float (Sea-Bird
Electronics) to integrate the microstructure float (Figure 1).
The measured along front flow of the Kuroshio Extension
show a strong eastward current exceeding 1.5 ms-1 near the
surface (Figure 2a). The across front flow is computed as
the normal velocity component with respect to the Kuroshio
axis, which is defined as the 500-m mean 30 hour lowpass
filtered velocity obtained by the EM-APEX float. The computed
across front velocity shows alternating positive and negative
bands modulating with near-inertial frequencies (Figure 2b),
indicating the strong influences from the near-inertial internal
waves Thermohaline interleaving structures are observed in
the across front Underway-CTD survey, with a characteristic
cold low salinity tongue extending from north along isopycnal
at σθ=26.25 (Figure 2c). These interleaving structures are also
found below 150 m depth, over 900 km along the Kuroshio
Extension Front (Figure 2d and f). Spectral analyses show that
along isopycnal rate of change in spiciness and vertical shear
are correlated at subinertial and near-inertial frequencies,
suggesting that the observed interleaving structures are
modulated by subinertial and near-inertial shear (Nagai et
al. 2015). Measured microscale thermal variance dissipation
rates χ are very large O(10-7-10-6 K2s-1) between 150-400 m
(Figure 2i) where the thermohaline interleaving structures are
observed, while turbulent kinetic energy dissipation rates ε
are relatively small O(10-10-10-9 W kg-1) (Figure 2h). The trend
that large χ and small ε is the indication of the possibility of
the double-diffusive convection. Using Osborn model (Osborn
1980) and Osborn-Cox model (Osborn and Cox 1972), the
turbulent eddy diffusivity for density and effective thermal
diffusivity are computed by Kρ=γε/N2, and Kθ=χ/2(Tz)2,
respectively, where γ is efficiency factor, 0.2, N buoyancy
frequency, Tz background vertical temperature gradient.
Figure 1: The microstructure float. A MicroRider (RSI Victoria Canada)
is mounted on a Navis-Float (SBE).
The microstructure float carries two shear probes and two
Fp07 thermistor sensors, to measure microscale vertical shear,
and microscale temperature gradient at 512 Hz, respectively.
The EM-APEX float measured absolute horizontal current
velocity, temperature and salinity about every 2 hours for
500 m water column, and the microstructure float measured
microstructures, temperature and salinity every 4 hours for
500 m water columns. Deployments of these two floats in the
main stream of the Kuroshio Extension allowed us to obtain
high-resolution velocity, microstructure, and hydrographic
data along the Kuroshio Extension, while these floats traversed
the meandering front rapidly to the downstream (Figure 2g).
The microstructure float was recovered after about 3 days,
while the EM-APEX float was kept profiling for over 10 days.
Across the Kuroshio Extension Front, where two profiling floats
were deployed, temperature, salinity, and current velocity were
measured using a shipboard ADCP (38KHz Teledyne RDI) and
a tow-yo CTD (Underway-CTD, Teledyne Oceanscience).
Turbulent kinetic energy dissipation rates ε are computed
by integrating the turbulent shear spectra where the spectra
agree with the Nasmyth model spectrum (Nasmyth 1970).
The shear spectra are converted to wave number spectra
using the free-rising speed of the float, 0.11-0.26 ms-1. Noises
from the buoyancy pump of the float typically contaminate
the turbulent shear signal for the bottom 100 m, and appear
at frequencies higher than 30-40 Hz; moderate levels of
turbulence, O(>10-9 Wkg-1), are not likely affected. The
microscale thermal variance dissipation rate χ is obtained by
integrating temperature gradient spectra for the range without
electronic noise at higher frequency. With slow rising speeds
of the float, the temperature gradient spectra show excellent
agreement with Kraichnan spectrum (Kraichnan 1968).
Figure 2: Across-frontal depth-latitude plots for (a) ADCP alongfront flow (ms-1), (c) potential temperature (oC), and (e) salinity (PSU)
measured by Underway-CTD along the observation line shown as solid
black line in (g). Along-frontal depth-time plots of (b) across-frontal
flow (ms-1), (d) potential temperature (oC), (f) salinity (PSU) measured
by EM-APEX float along the Kuroshio Extension Front for 10 days.
White boxes in (d and f) are time-depth ranges for (h) the turbulent
kinetic energy dissipation rates ε and (i) microscale thermal variance
dissipation rates χ measured by microstructure float.
14
Average effective thermal diffusivity Kθ between 150 and 400
m depth is 1.7 x 10-3 m2s-1, which is 100 times larger than that
for turbulent eddy diffusivity for density, Kρ of O(10-5 m2s1
). Computed Turner angles (Ruddick 1983) suggest that
these large effective thermal diffusivity and χ with relatively
small eddy diffusivity for density Kρ and ε are found with
double-diffusive favourable conditions (Nagai et al. 2015b).
The derived effective thermal diffusivity is found to be
consistent with the previous parameterizations for doublediffusive convection (Radko and Smith 2012; Fedorov 1988;
Nagai et al. 2015b), suggesting further that the observed
large thermal diffusivity is caused by double-diffusion.
In this study we observed thermohaline interleaving structures
below 150 m depth along the Kuroshio Extension Front over
900 km with double-diffusive favourable conditions. Because
the high correlations between vertical shear of subinertial and
near-inertial frequencies and rate of change in spiciness are
found, double-diffusive favourable thermohaline interleaving
structures are likely to be formed both by subinertial and
near-inertial shear. Within these thermohaline interleaving
layers, larger microscale thermal variance dissipation rates
χ~O(10-7-10-6 K2s-1) and effective thermal diffusivity Kθ of
O(10-4 -10-3 m2s-1) are observed with small turbulent kinetic
energy dissipation rates ε~ O(10-10-10-9 W kg-1) and turbulent
eddy diffusivity for density Kρ of O(10-5 m2s-1). The observed
effective thermal diffusivity Kθ is found to be consistent with
the previous double-diffusion parameterization, suggesting
that the large microscale thermal variance dissipation rate χ
is caused by double-diffusive convection. Accordingly, our
results suggest that mesoscale subduction, obduction and
near-inertial waves near oceanic fronts have catalytic effects
to enhance subsurface double-diffusive convection. The
enhanced thermal dissipation below the Kuroshio Extension
with interleaving thermohaline structures suggests that
double-diffusive convection is an important agent to dissipate
tracer variance at microscales in subsurface layers of the
Kuroshio Extension Front.
Acknowledgments
We thank Capt. Inoue, crews of R.V. Kaiyo (JAMSTEC), Mr.
Okada at NME Ltd. for assists in field surveys, all those who
participated in the cruise including Dr. Lueck at RSI, Dr. Li,
Dr. Kokubu, Mr. Mabuchi at JFE-Advantech, Dr. Hasegawa at
FRA, all the students and staff of the lab (Homma, Masunaga,
Foloni-Neto, Yukawa, Nishi, Furuyama, Takeuchi, Nakamura,
Sugata, Hohman, Allmon), Dr. Hosoda, Ms. Hirano and Mr.
Nakajima at JAMSTEC, Mr. Dunlap at APL, and Dr. Mitchell and
Mr. Quittman at SBE for the float configuration and operations,
Dr. Wolk and Mr. Stern at RSI, and Mr. Yazu at JFE-Advantech
for the MicroRider integrations, Ms. Lynn Allmon for editorial
assistance, and Prof. Ruddick and Dr. Schmitt for their
insights. TN thanks JSPS (KAKENHI 24684036), “”The Study
of Kuroshio Ecosystem Dynamics for Sustainable Fisheries
(SKED)”” funded by MEXT, MIT-Hayashi Seed Fund. AT
thanks NSF-1434512 and ONR-N000141310456 for support.”
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Whitt, D., and L. Thomas, 2013: Near-inertial waves in strongly
baroclinic currents, J. Phys. Oceanogr., 43, 706–725.
Chen1, Lixiao Xu1, Xue Liu2,1, Zhao
Jing2,1,Lixin Wu1, Dexing Wu1, Ping
Chang2,1
1. Physical Oceanography Laboratory/CIMSST,
Ocean University of China and Qingdao National
Laboratory for Marine Science and Technology,
Qingdao , China
2. Department of Oceanography, Texas A&M
University
Introduction
The Kuroshio extension region has been identified as a key
location in the extratropics in the North Pacific where sharp
oceanic fronts and energetic mesoscale oceanic eddies induce
considerable sea surface temperature (SST) variability,
leading to intensive air-sea interaction. Understanding these
frontal- and mesoscale air-sea interaction has important
implications to improve future climate modelling, prediction
and application at the midlatitude. However, quantifying
and understanding this air-sea coupling have proven to be
formidable challenges for a number of reasons. First, as one
of the strongest western boundary currents, the Kuroshio
extension is the region where both atmosphere and ocean are
dynamically unstable, producing energetic eddies that tend to
mask the ocean-atmosphere feedback signals. Second, unlike
the tropics where ocean-atmosphere interaction occurs in
basin-to-global scale, the interaction in the Kuroshio extension
region takes place along narrow frontal zones where the SST
gradient is strong. There is generally a lack of understanding
of small-scale processes in the ocean and atmosphere. Third,
until very recently, available observations and numerical
modelling tools were inadequate to resolve such small-scale
dynamical processes in western boundary current regimes,
making it difficult to further investigate the frontal- and
mesoscale air-sea interaction. As a contribution to the ongoing
effort, we aim to develop a sustainable observational network
16
through an international cooperation, establish a theoretical
framework of the multi-scale ocean-atmosphere interactions
in the Kuroshio extension region using eddy-resolving highresolution coupled regional climate model, reveal key physical
processes in determining climate changes, and improve
the ocean and climate predictability in the North Pacific.
Observational Network
Until recently, there have been quite limited high-resolution
observations in the Kuroshio extension region. One project
providing systematic observational data is the Kuroshio
Extension System Study (KESS), launched from 2004 to
2006 and funded by US National Science Foundation (NSF),
which deployed an observing array consisting of Argo profiling
floats, moored-profiler, moorings, Current-Pressure equipped
Inverted Echo Sounder (CPIES) as well as a surface buoy (KEO)
located south of the Kuroshio (Figure 1). North of the Kuroshio,
another buoy (JKEO) was deployed by the Japan Agency for
Marine-Earth Science and Technology (JAMSTEC)’s Institute
in 2007. However, JKEO has been discontinued after the end
of the Japanese HOTSPOT project in 2015. Following KESS
and Japanese HOTSPOT projects, a new observational plan
in the Kuroshio extension region under Intergovernmental
Oceanographic Commission (IOC) Sub-Commission for
the Western Pacific (WESTPAC), Air-Sea Interaction in the
Kuroshio Extension and its Climate Impact (AIKEC) led by the
Ocean University of China (OUC) and the Texas A&M University
(TAMU), was setup to maintain continuous and sustainable
observations in this region. Supported by the Ministry of
Science and Technology of China, the Natural Science
Foundation of China and the Qingdao National Laboratory for
Marine Science and Technology, systematic observations in
the Northwestern Pacific Ocean are being carried out during
four cruises from 2014 to 2016 conducted by OUC. A summary
of these cruises is shown in Figure 1. Two moorings have been
deployed during the past two years: south of the Kuroshio
extension where most of cold oceanic eddies occur, the M1
mooring has been placed near NOAA KEO buoy, so that the flux
measurement can be combined with oceanic measurements
to quantify the importance of interaction between oceanic
eddy and atmosphere in this region; north of the Kuroshio
extension where most of warm oceanic eddies and the most
intensive mesoscale air-sea couplings occur, the M2 mooring
has been deployed in the 2016 cruise, where a new surface
flux buoy, named CKEO, will be placed in the following year so
that mesoscale air-sea interaction dynamics in the region are
regarded as a contrast with those in the south of the Kuroshio
extension. In our future plan, CPIES will be deployed near M1
and M2 moorings to measure the high-resolution current,
which assist to study the energy cascading of the small scale
oceanic eddies. In addition, gliders will be flown repeatedly
between M1 and M2 moorings in the future cruises.
With respect to the completed cruises, we have done a lot
of work in the Kuroshio extension region. The 2014 research
cruise took place from 17 March to 23 April 2014, and it was
carried out by the research vessel “Dong Fang Hong 2”. In
the 38-day voyage, researchers deployed 25 sets of Argo
floats and, for the first time in China, a deep-sea (6000m)
mooring system (M0) in the Kuroshio extension region (Figure
1). In November 2015, another deep-sea mooring (M1) with
McLane Moored Profiler (MMP ) mounted was deployed and
successfully recovered in the 2016. The 2016 cruise also
deployed another subsurface mooring at the same location
(M1) and a new subsurface mooring in the north of the Kuroshio
Extension in the east of JKEO (M2). In addition to subsurface
moorings, hundreds of radiosondes and ConductivityTemperature-Depth (CTD) casts have been conducted
during the previous cruises to profile the atmospheric
17
Figure 1: Moorings, Buoys and CTD/Argo/MSS positions in the new
observation plan.
boundary layer, lower troposphere and the upper-layer ocean.
Numerical Models
A Coupled Regional Climate Model (CRCM) (Ma et al. 2016)
has been developed by TAMU and OUC over the past several
years, leveraging existing community model development
activities, particularly the climate version of the Weather
Research Forecast (WRF) Model developed at the US National
Center for Atmospheric Research (NCAR), and the Regional
Ocean Modeling System (ROMS) developed at Rutgers
University and University of California - Los Angeles (UCLA).
The use of these models offers a standardized means for
coding and reporting and will greatly facilitate sharing results
with the broader climate modelling community. Within the
coupler, the atmosphere and ocean exchange surface fluxes at
each coupling step. In current model configuration in the North
Pacific, the surface fluxes are computed by WRF using modified
bulk drag and enthalpy coefficients to improve accuracy in
high-wind regimes. Surface stress formula is also modified to
account for ocean surface velocity. Air-sea coupling frequency
is typically set at one hour in the standard configuration to
resolve diurnal cycle, but higher or lower frequencies may be
used. The model includes a novel feature of spatially filtering
SST at each coupling step that allows suppressing frontaland mesoscale air-sea coupling. CRCM and its component
models can be forced with observed data sets or global model
simulation outputs at the lateral boundaries to produce an
estimate of the climate state at high spatial resolution. The
model has been widely tested in our study and applied in
western boundary current regions including the Kuroshio
extension in the North Pacific and the Gulf Stream in the North
Atlantic. Results of the simulations are very encouraging and
produce realistic Kuroshio (Gulf Stream) path, oceanic eddies
and storm track variability in the North Pacific (Atlantic). In
addition, the CRCM model shows great ability in forecasting
the path of Hurricane Isaac in 2012.
Research Activities
Three research projects related to the study of the Kuroshio
extension were funded in China, focusing on the response
to global warming and regulation of climate change by
the Pacific and Indian Oceans, multiscale variations,
mechanisms and predictability of the Northwest Pacific
Ocean, and climatic impacts of ocean-atmosphere
interaction in the Kuroshio and its extension region.
During the Pacific Mode-Water Ventilation Experiment
(P-MoVE; Xie 2013), 17 Argo profiling floats with enhanced
daily sampling were deployed in an anticyclonic eddy
region south of the Kuroshio extension in late March 2014.
More than 3,000 hydrographic Argo profiles following the
anticyclonic eddy were obtained, offering an unprecedented
detailed observational view of eddy subduction process in the
region for the first time (Xu et al. 2016).
To understand the role of extreme heat flux events associated
with atmospheric synoptic storms in frontal-scale air-sea
interactions in the western boundary current regions, and
its relationship with large-scale atmospheric variability, we
have completed a comprehensive analysis of boreal winter
extreme flux events in the Kuroshio Extension Region (KER)
of the Northwestern Pacific and the Gulf Stream Region
(GSR) of the Northwestern Atlantic, based on reanalysis
and observationally data sets (Ma et al. 2015a). The results
show that there is a close relationship between the extreme
flux events over KER/GSR and the East Atlantic Pattern
(EAP)/Pacific Decadal Oscillation (PDO), with more/less
frequent occurrence of the extreme flux events during a
positive/negative EAP/PDO phase. Moreover, ALP/EAP
can be explained as accumulated effects of the synoptic
winter storms accompanied with the extreme flux events.
To investigate the extent that frontal- and mesoscale SST
associated with ocean mesoscale eddies can impact the
atmosphere locally and remotely (Figure 2.), we have
completed a suite of high-resolution regional climate
model WRF simulations in North Pacific domain at various
spatial resolutions ranging from 9 km, 27 km and to 162 km.
Mesoscale SST variability associated with oceanic eddies not
only can have a major impact within and above the atmosphere
boundary layer locally (Figure 2.), but more importantly can
exert remote influence on basin-scale atmosphere circulation.
Mesoscale SST variability, though largely confined in the
Kuroshio extension region, can affect local cyclogenesis and
downstream storm development, which further exert a
significant distant influence on winter rainfall variability along
the U. S. Northern Pacific coast, as well as a remote barotropic
mean flow response in the eastern North Pacific. A new theory
on midlatitude SST influence on the atmosphere has been
formulated based on the concept of moist baroclinic instability:
the presence of mesoscale SST anomalies enhances the
diabatic conversion of latent heat energy to transient eddy
energy, intensifying winter cyclogenesis via moist baroclinic
instability, which in turn affects downstream stormtrack
and precipitation (Ma et al. 2015b). This finding points to
the potential of improving forecasts of extratropical winter
cyclones and storm systems and projections of their response
to future climate change, which are known to have major social
and economic impacts, by improving the representation of
ocean eddy–atmosphere interaction in forecast and climate
models.
Summary and Work Underway
Major progress has been made during the last three years
(2012-2015), including three Chinese national research
projects focusing on the air-sea interaction study in the Kuroshio
extension region and two research cruises in the Northwestern
Pacific Ocean. The accomplishment of these activities not only
improves our understanding of frontal- and mesoscale air-sea
interaction at midlatitude and provides scientific support for
the enhancement of prediction capability of climate models,
but also promotes interdisciplinary integration as well as
international communication and cooperation in observation
and research in the western boundary current regions.
The 2016 research cruise has taken place from March 22 to
May 3 to deploy another deep-sea mooring, while a newlydeveloped buoy CKEO north of the Kuroshio will be deployed
in the following year. However, international collaborations are
highly desired to perform long-term mooring maintenance. The
CKEO flux buoy will be built as a joint venture between China
and NOAA/PMEL’s or GERG/TAMU. Data collection during
these regular cruises and the mooring data will contribute
to build a systematic observation network in the Kuroshio
extension region in the future.
There are also several important scientific questions related
to frontal- and mesoscale air-sea interaction in the Kuroshio
extension region and will be addressed in a future study. First,
we intend to investigate the feedback mechanism governing
ocean mesoscale eddy-atmosphere interaction and its role in
affecting large-scale atmosphere and ocean circulation. This
has been done by conducting an ensemble of high-resolution
eddy-resolving CRCM simulations for both the atmosphere
and ocean, with and without mesoscale SST signatures in the
North Pacific. Second, we will carry out Lagrangian analyses
of ocean mesoscale eddies in the Kuroshio extension regions
using an objective eddy-recognition algorithm. These analyses
will examine atmospheric responses following the identified
ocean eddies and its feedback on oceanic eddies, and compare
the responses between warm and cold ocean eddies. These
analyses can also help address whether the atmospheric
responses to warm and cold eddies are asymmetric, leading to
a rectified effect of eddies on the atmosphere. Finally, we will
attempt to construct simple empirical models of the mesoscale
ocean-atmosphere feedback based on high-resolution coupled
simulations, and parameterize its effect in coarse-resolution
climate models used for long-term integrations, in order to
reduce the climate model bias by correctly representing the
mesoscale air-sea coupling.
Figure 2: Winter season mean (NDJFM) (a) spatially high-pass filtered SST (contour, °C) and turbulent heat flux (color shaded, w2/m) (b) spatially highpass filtered SST (contour, °C) and convective rainfall (color shaded, mm/day) derived from 10 members ensemble mean of 27km WRF simulations.
18
References
Ma, X., Z. Jing, P. Chang, X. Liu, R. Montuoro, R. J. Small, F.
O. Bryan, R. J. Greatbatch, P. Brandt, D. Wu, X. Lin and L. Wu,
2016: Western Boundary Currents regulated by interaction
between ocean eddies and the atmosphere. Nature, Accepted.
Ma, X., P. Chang R. Saravanan, D. Wu, X. Lin, L. Wu, 2015a:
Winter Extreme Flux Events in the Kuroshio and Gulf Stream
Extension Regions and Relationship with Modes of North
Pacific and Atlantic Variability, J. Clim., 28, 4950-4970. doi:
http://dx.doi.org/10.1175/JCLI-D-14-00642.1
Ma, X. H., P. chang, R. Saravanan, M. Raffaele, J. S. Hsieh,
D. Wu, X. Lin, L. Wu and Z. Jing, 2015b: Distant Influence of
Kuroshio Eddies on North Pacific Weather Patterns? Scientific
Reports ,5, 17785 ,doi:10.1038/srep17785.
Xie, S.-P., 2013: Advancing climate dynamics toward reliable
regional climate projections. Journal of Ocean University of
China, 12, 191-200.
Xu, L., P. Li, S.-P. Xie, Q. Liu, C. Liu, and W. Gao, 2016:
Observing mesoscale eddy effects on mode-water subduction
and transport in the North Pacific. Nature communications, 7.
Effects of Deep
Bottom Topography
on the Kuroshio
Extension Studied by
a Nested-grid OGCM
Masao Kurogi1, Yukio Tanaka, and
Hiroyasu Hasumi2
Technology, Yokohama, Japan
2. Atmosphere and Ocean Research Institute, The
University of Tokyo, Kashiwa, Japan
Introduction
The Kuroshio is the western boundary current of the North
Pacific subtropical gyre. After separating from the coast of
Japan, its extension is called the Kuroshio Extension (KE).
There are two quasi-stationary meanders in the KE path that
can be seen in Figure 1a. Eddy activity in the KE region is high
(Figure 1b) and the KE system is known to oscillate between
stable states with low eddy activity and unstable states with
high eddy activity on the decadal scale (Qiu and Chen, 2005).
Previous studies pointed out the effects of bottom topography
on the KE path states. In the upstream region, the Kuroshio
and KE flow on the Izu-Ogasawara Ridge and deep trench
just east of it (see Figure 2a). Qiu and Chen (2005) pointed
out that the KE path becomes unstable when the KE migrates
southward and the KE inflow rides over shallow region of
the Izu-Ogasawara Ridge. East of the trench, there is a
vast abyssal plain with gradual undulation and seamounts.
According to Hurlburt et al. (1996), such abyssal plain and
seamounts are important for formation of the KE quasistationary meanders. Downstream of this region, the KE
encounters the Shatsky Rise. Qiu and Chen (2010) suggested
the influence of the Shatsky Rise on the KE path. That is, when
the KE migrates southward and rides over shallow portion of it,
disturbance is generated and the upstream KE is destabilized.
The maximum depth widely used in eddy-resolving simulations
of (quasi) global Ocean General Circulation Models (OGCMs)
lies mostly between 5500 m and 6000 m (e.g., Maltrud and
McClean, 2005; Sasaki et al., 2008; Maltrud et al., 2010).
Setting the maximum depth in models at 5500 m is enough to
represent characteristic topography such as the Izu-Ogasawara
Ridge, Shatsky Rise, and seamounts in the KE region. However,
the undulation of vast abyssal plain mentioned above cannot
be represented. The characteristic wavelength of large-scale
abyssal undulation in this region is roughly 500-1000 km. As
pointed out by Treguier and McWilliams (1990), it is expected
that the effect of bottom topography decreases with the
e-holding scale of f/NK, where f is the Coriolis parameter, N is the
Brunt-Väisälä frequency, and K is the horizontal wavenumber.
A rough estimate of this scale implies that the effects of such
19
large-scale topography penetrate to the upper ocean. In this
study, the effects of such deep bottom topography on the KE
are investigated by using a nested-grid eddy-resolving OGCM.
Description of Model and Experiments
The model used in this study is a two-way nested-grid OGCM
based on an ice-ocean coupled model, named COCO (Hasumi,
2006). An inner model of the western North Pacific region
(116°E-172°W, 14.3°N-52.9°N) is nested into the outer model
covering the whole North Pacific (105°E-75°W, 14.8°S-67.7°N).
The horizontal resolution of inner and outer models is
0.1°×0.1°cosθ and 0.5°×0.5°cosθ, respectively, where θ is the
latitude. There are 74 vertical levels with thickness increasing
from 5 m at the surface to 200 m at the maximum depth of
9200 m.
The two-way nesting method adopted here is basically the
same as that of Kurogi et al. (2013) with exception of the
following two improvements. Firstly, a two-way coupling
between the models is realized for all processes including
calculations of sea ice. This enables smooth connection of sea
ice variables across the interfaces between the models and
stable calculation of sea ice. The second improvement is the
implementation of conservative method similar to Debreu et
al. (2012). At the grid interface, flux of an outer model grid is
fitted to the value obtained by summing fluxes of inner model
grids facing the same range of the interface. By applying such
technique in realistic settings, Debreu et al. (2012) could not
stably integrate the model with a terrain-following vertical
coordinate. Our model with (basically) the geopotential height
vertical coordinate was successful in performing a stable
integration with realistic setting as described above.
Two experiments are executed by changing bottom
topography. For the first case, deep bottom topography to the
depth of 9200 m is represented (Figure 2a, hereafter called as
EX9200). In the second case, bottom topography only to the
depth of 5450 m is represented (hereafter called as EX5450).
In this case, most part of the KE region upstream of the
Shatsky Rise is flat (Figure 2b). For both experimental cases,
the model is integrated from 1950 to 2009 with CORE Ver. 2
Interannual forcing (Large and Yeager, 2009). Relative wind
is used to calculate wind stress for realistic representation of
eddy activities in the Kuroshio and KE region (Tsujino et al.,
2013). Results of the last 16 years (1993-2008) are compared.
Effects of deep bottom topography on the KE
Deep bottom topography in the KE region has a great impact
on the time-averaged sea surface height (SSH) field. In
EX9200 with deep topography (Figure 2c), the zonal location
of crests of the KE meanders (around 144°E, 150°E) and the
meander amplitude (about 130 km, defined here by amplitude
in latitudinal direction of the quasi-stationary meanders) are
close to observations (Figure 1a), though the simulated KE
flows about 0.5-1° northward compared to observations. The
zonal location of crests of the KE meanders is almost the same
in EX5450 without deep topography (Figure 2d). However, the
amplitude of the KE meanders in this case is decreased by half.
Deep bottom topography also affects the strength and
horizontal distribution of the eddy activity. The regional average
(141°E-165°E, 30°N-40°N) of root-mean-square (RMS) of sea
surface height anomaly (SSHA) (bottom panels of Figure 2) is
13 % larger in EX9200 (21.4 cm) compared to EX5450 (18.9
cm), and the former is closer to observations (22.4 cm). RMS
of SSHA reaches maximum around 146° E and decreases
gradually to the east in observations (Figure 1b). The regional
average of RMS of SSHA in downstream region (153°E-165°E,
30°N-40°N) is 13 % smaller than that in the upstream region
(141°E-153°E, 30°N-40°N) for observations. For the simulated
KE, the regional average in the downstream region tends to be
larger compared to that in the upstream region especially for
EX5450 (13% larger) than for EX9200 (4% larger). Further,
the region with high RMS of SSHA in EX5450 (Figure 2f) is
much narrower than in observations (Figure 1b) and EX9200
(Figure 2e). As a whole, simulated distribution of RMS of SSHA
is closer to the observations in the case with deep bottom
topography.
The upper panels of Figure 3 show the upstream KE path length
between 141°E and 153°E. This length is introduced by Qiu and
Chen (2005) as an index of stability of the KE and shorter
(longer) path length indicates stable (unstable) KE states.
The upstream KE path length is shorter in EX5450 (1352 ± 219
km) than in EX9200 (1593 ± 319 km) and observations (1743
± 465 km), reflecting the difference of eddy activity that RMS
of SSH of the upstream region is higher in observations (23.9
cm) and EX9200 (21.0 cm) than in EX5450 (17.8 cm). Shorter
path length in EX5450 is also due to the reduced amplitude of
the quasi-stationary meanders because the upstream KE path
length for time-averaged SSH (green line in Figure 1a, 2c and
2d) is 2-7% smaller in EX5450 (1187 km) than in EX9200 (1276
km) and observations (1214 km).
Time variation of the upstream KE path length is different
between experimental cases. There is interannual variability
in the upstream KE path length in EX9200 (Figure 3b) though
the phase does not agree well with observations (Figure 3a).
The upstream KE path length in EX5450 is almost shorter
than observational time-averaged value and variation is much
smaller than observations and EX9200 except around 2007
(Figure 3c). In this case, stable states are dominant. The
middle panels of Figure 3 show the spaghetti diagram of the
KE path for the year in stable states. The upstream KE path in
stable states is similar to observations though the amplitude
of the KE meander in EX5450 (Figure 3f) is smaller compared
to observations (Figure 3d) and EX9200 (Figure 3e). For
unstable states of the KE, the simulated KE path (Figure 3h and
3i) in the upstream region is convoluted compared to stable
state (Figure 3e and 3f) but less convoluted to observations
(Figure 3g).
Figure 1: (a) Time-averaged SSH and (b) RMS of SSHA during 19932008 for observations with altimeter data. Contour interval in (a) is 10
cm. Green line in (a) indicates the KE axis defined by contour line of SSH
with value of its regional average (141°E-153°E, 30°N-40°N).
Summary and discussion
It is found that deep bottom topography has a profound effect
on the KE. By representing bottom topography deeper than
20
Hurlburt, H. E., A. J. Wallcraft, W. J. Schmitz, P. J. Hogan,
and E. J. Metzger, 1996: Dynamics of the Kuroshio/Oyashio
current system using eddy-resolving models of the North
Pacific Ocean, J. Geophys. Res., 101, 941–976.
Figure 2: Bottom topography (top panels), time-averaged SSH (middle
panels), and RMS of SSHA (bottom panels) for experimental cases. Left
and right panels are for EX9200 and EX5450, respectively. Contour
interval in (a-b), (c-d) are 1000 m, 10 cm, respectively. Green line in (cd) indicates the KE axis.
5450 m, the amplitude of the KE quasi-stationary meanders
and RMS of SSHA are enhanced. The upstream KE path length
also becomes longer with its variability enhanced in the case
with deep bottom topography. The detailed mechanism how
deep bottom topography affects the KE is under investigation.
To better represent and understand the KE, there are
remaining issues. For example, the simulated KE in unstable
states is less convoluted compared to observations. This may
be related to northward positioned path of the simulated KE.
The KE tend to flow north of shallow potion of the Shatsky Rise
which is pointed
out to destabilize the KE (Qiu and Chen, 2010). Effects of shallow
portion of the Shatsky Rise will be clear in an experiment with
this shallow region is extended northward to interact the KE
jet. We are planning to execute such additional experiments to
better understand the effects of bottom topography on the KE.
Acknowledgements
Figure 1 and left panels of Figure 3 are drawn using the
altimeter products produced by Ssalto/Duacs and distributed
by Aviso, with support from Cnes. This work is supported by
Strategic Programs for Innovative Research (SPIRE) Field
3 of the Ministry of Education, Culture, Sports, Science, and
Technology (MEXT), Japan. This work is also supported
by the FLAGSHIP2020, MEXT within the priority study4
(Advancement of meteorological and global environmental
predictions utilizing observational “Big Data”). Numerical
simulations were conducted by using computational
resources of the K computer provided by the RIKEN
Advanced Institute for Computational Science through
the HPCI System Research project (Project ID: hp120279,
hp130010, hp140219, hp150213, hp150287, and hp160230).
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Two-way nesting in split-explicit ocean models: Algorithms,
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Hasumi, H., 2006: CCSR Ocean Component Model (COCO)
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21
Figure 3: The length of KE path (axis) between 141°E and 153°E (top
panels). The KE path (axis) for every 14 days in the year with the
shortest KE path length (middle panels) and the longest KE path length
(bottom panels). Corresponding year is indicated in the each panel.
Left, middle and right panels are for observations with altimeter data,
EX9200 and EX5450, respectively. Red and blue (dotted) lines in the
top panels indicate low-pass filtered path length with time scale longer
than 356 days and time-averaged path length for observations (each
experimental case), respectively.
Kurogi, M., H. Hasumi, and Y. Tanaka, 2013: Effects of
stretching on maintaining the Kuroshio meander. J. Geophys.
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An Extra Product
of the JRA-55
Atmospheric
Reanalysis and In
Situ Observations in
the Kuroshio-Oyashio
Extension Under the
Japanese “Hotspot
Project”
Hisashi Nakamura1,2, Yoshimi Kawai2,
Ryusuke Masunaga1, Hirotaka
Kamahori3, Chiaki Kobayashi3 and
Makoto Koike4
1. Research Center for Advanced Science and
Technology, The University of Tokyo
Technology, Yokohama, Japan
3. Meteorological Research Institute, Japan
Meteorological Agency, Tsukuba, Japan
4. Department of Earth and Planetary Science,
The University of Tokyo, Tokyo, Japan
Introduction
It is well established that the extratropical ocean tends to
respond passively to mechanical and thermal forcing by
the variability in the overlying atmosphere generated locally
by internal dynamics or remotely through atmospheric
teleconnection from the Tropics. Basin-scale atmospheric
anomalies are maintained under the feedback forcing by
synoptic-scale cyclones and anticyclones that recurrently
develop along an oceanic stormtrack. The associated
anomalous surface winds drive basin-scale anomalies in seasurface temperature (SST) by modulating surface latent and
sensible heat fluxes (hereafter LHF and SHF, respectively)
and/or entrainment at the mixed-layer bottom. Over the
North Pacific, the El Niño/Southern Oscillation (ENSO)
leaves distinct imprints on SST by remotely modulating the
stormtrack activity as “atmospheric bridge” (e.g., Alexander
et al. 2002). During an El Niño event, for example, stronger
surface westerlies yield negative SST anomalies over the
central Pacific due to enhanced entrainment and heat release
into the atmosphere.
Nevertheless, the passiveness of the extratropical ocean
sounds rather counterintuitive if a climatic role of subtropical
ocean gyres is considered in carrying a huge amount of heat
from the Tropics. The transported heat is then released into
the midlatitude atmosphere through LHF and SHF intensively
from narrow western boundary currents (WBCs), including
the Kuroshio. Under the strong influence of the East Asian
winter monsoon, the Kuroshio, its eastward Extension and the
Tsushima Current climatologically release as much as 2PW
of sensible and latent heat into the atmosphere in January
(Nakamura et al. 2015), which is twice as much as from the
Gulf Stream (0.9PW), exerting thermodynamic forcing on the
overlying atmosphere (Kelly et al. 2010; Kwon et al. 2010).
Thus the western North Pacific, including the adjacent marginal
seas, can be recognized as the most pronounced climatic
“hot spot” among the global WBCs (Nakamura et al. 2015).
This maritime domain is also characterized by the Subarctic
Frontal Zone (SAFZ) in the Kuroshio-Oyashio Extension (KOE),
a prominent oceanic frontal zone with strong SST gradient
that is influential on the stormtrack formation (Nakamura et al.
2004). Persistent SST anomalies generated by decadal-scale
SAFZ variability can force basin-scale atmospheric anomalies,
with the anomalous surface Aleutian Low, by modulating
stormtrack activity (Frankignoul et al. 2011; Taguchi et al.
2012). Aiming at deepening our understanding of oceanic and
atmospheric processes occurring along the Kuroshio, in the
KOE region and East Asian marginal seas, a project “MultiScale Air-Sea Interaction under the East-Asian Monsoon: A
‘Hot Spot’ in the Climate System” was funded from summer
2010 to spring 2015 by the Japanese Ministry of Education,
Culture, Sports, Science and Technology (Nakamura et al.
2015). In this so-called “Hotspot project”, a total of a hundred
scientists and graduate students from both the Oceanography
and Meteorological Societies of Japan participated. By
unifying advanced high-resolution numerical modeling on
the Earth Simulator and new-generation satellite data and
by conducting in situ observation campaigns, the project
contributed substantially to deepening our understanding of
multi-scale interactive processes involved actively in the airsea heat and freshwater exchanges and their influence on
the climate variability. In the following, two data sets as main
outcomes from the project are introduced to the community.
Extra product of the Japanese reanalysis of
the global atmosphere
Wide spectrum of multi-scale air-sea interaction within the
“hot spots” has been revealed in recent satellite data and
high-resolution numerical modeling. For example, satellite
scatterometer measurements have revealed mesoscale
patterns of surface wind convergence/divergence generated
under the background winds blowing across frontal SST
gradient (Xie 2004; Chelton et al. 2004; O’Neill 2010). On
the warmer side of a SST front along a warm WBC, turbulent
mixing of wind momentum is enhanced, and cross-frontal
winds therefore act to generate surface convergence or
divergence. Enhancement and suppression of heat release
from the ocean on the warmer and cooler sides of the SST front
also yield surface convergence and divergence, respectively,
through hydrostatically lowering and raising surface pressure
(Small et al. 2008; Shimada and Minobe 2011; Tanimoto et
al. 2011). The surface convergence thus generated yields
ascending motion at the top of the boundary layer, acting to
enhance cloudiness and precipitation locally, for example,
along the warm KE in winter (Tokinaga et al. 2009; Masunaga
et al. 2015). Particularly high SST along the Kuroshio in the
East China Sea can develop deep convective clouds, especially
under the warm, moist monsoonal airflow from the Tropics
(Sasaki et al. 2012; Miyama et al. 2012). In situ observations
can barely identify the climatological signatures or snapshots
22
of these mesoscale atmospheric imprints of the Kuroshio
and KE (Tokinaga et al. 2009; Tanimoto et al. 2011), but
the spatial and temporal resolutions of the data are too
coarse to investigate the variability of those imprints.
For this purpose a long-term atmospheric reanalysis dataset
may be suited. However, its usefulness depends critically on
the resolution not only for the forecast model used for data
assimilation but also for the SST data used as the model lowerboundary condition. In this regard, the ERA-Interim (Dee et
al. 2011) is unique, in which the resolution of the SST data
used has been improved at the beginning of 2002. Masunaga
et al. (2015, 2016a,b) have recently pointed out that those
mesoscale atmospheric imprints of the KE as mentioned
above are well represented climatologically in the ERA-Interim
only in the period since 2002 and so are their modulations
forced by the KE variability, while the corresponding imprints
are virtually missing before 2002. Their findings indicate that
circulation and thermal structure in the boundary layer is not
strongly constrained by observations and their representation
in reanalysis (or operational analysis) can therefore be
highly sensitive to SST data prescribed in data assimilation.
a)
b)
resolution, which is coarser than that of the forecast model
and insufficient for representing fine-scale SST structure (Fig.
1a). In contrast, the extra product under construction, called
JRA-55CHS, utilizes the satellite-based MGDSST data in
place of COBE-SST used for the main product (“C” of JRA55C and JRA-55CHS stands for “Conventional”, whereas “HS”
for “High-resolution SST” and “Hot Spot”). The resolution of
MGDSST is quarter degree, which is sufficient for representing
frontal SST gradient associated, for example, with the KE
(Fig. 1b). Due to the limited availability of the MGDSST data,
JRA-55CHS, when completed, will become available for
the period since 1982. Unlike in the main JRA-55 product,
no satellite data for atmospheric variables are assimilated
into JRA-55CHS due to the availability of computational
a)
COBE-SST
b)
MGDSST
c)
Figure 1: Climatological wintertime (DJFM) SST (every 2°C) over the
North Pacific based on (a) COBE-SST and (b) MGDSST. Climatological
SST gradient (°C/100km) is colored. The climatology is defined for the
period from 1985 to 2010 except for 1999-2003, in which JRA-55CHS
is missing.
Motivated by Masunaga et al. (2015, 2016a,b), the
Meteorological Research Institute (MRI) and University of
Tokyo are now jointly preparing an extra product of a new
Japanese reanalysis of the global atmosphere (JRA-55). As
described in Kobayashi et al. (2015), the main product of
JRA-55 has been produced for the period since 1958 with a
state-of-the-art forecast model on T319 spectrum resolution
(equivalent to ~60 km resolution) with 60 vertical levels, in
which both satellite and other conventional measurements
(e.g., radiosonde data) were assimilated through the 4DVAR
method. As the lower-boundary condition for the JRA-55 main
product, the COBE SST data set (Ishii et al. 2005) was used
for the consistency over the entire data period. Based only
on in situ observations, COBE SST is archived on 1-degree
23
Figure 2: (a) Wintertime (DJFM) climatologies in MGDSST over the
North Pacific represented as local deviations (°C) from COBE-SST.
(b) As in (a), but for surface wind convergence (positive) /divergence
(negative) (10–5 s–1) represented in JRA-55CHS as local deviations from
that in JRA-55C. The SST difference shown in (a) is superimposed with
contours (every 0.3°C; dashed for negative), and climatological surface
winds in JRA-55CHS are plotted with arrows. (c) As in (b), but for total
cloudiness (%). The climatology is defined for the period from 1985 to
2010 except for 1999-2003, in which JRA-55CHS is missing.
resource. In this regard, JRA-55CHS can be compared in a
more straightforward manner with JRA-55C, another JRA-55
product that has been produced for the period since 1958 in the
same manner as the main product but without satellite data.
Figure 2a shows a difference map of climatological SST for
winter (December through March) between JRA-55CHS and
JRA-55C. Elongated dipolar SST differences apparent in Fig.
2a correspond to local SST fronts in the KOE region (Kida et
al. 2015), which are better represented in MGDSST for JRA55CHS than in COBE-SST for JRA-55C (Fig. 1). Relative to
JRA-55C, upward SHF in JRA-55CHS is locally enhanced and
suppressed on the warmer and cooler sides, respectively, of
each of those SST fronts, and so is upward LHF (not shown).
Compared to JRA-55C, JRA-55CHS features mesoscale
patterns of anomalous convergence/ divergence that
correspond to enhanced frontal SST gradient in MGDSST
(Fig. 2b). As a local response to enhanced SST gradient,
both the vertical mixing and hydrostatic pressure adjustment
mechanisms seem operative to yield the anomalous
convergence/divergence. Specifically, convergence is locally
enhanced right over the warm KE just off the east coast
of Japan, indicative of the dominant contribution from the
pressure adjustment mechanism. Otherwise, the signatures of
anomalous convergence/divergence appear to be displaced
systematically from the local SST anomalies but collocated
with the individual SST fronts that exhibit SW-NE tilts. This
is an indication of the vertical mixing mechanism acting on
the monsoonal northwesterlies. Crossing the fronts from
their cooler to warmer sides, the surface northwesterlies
tend to be accelerated by enhanced momentum mixing,
yielding divergence along the fronts. Likewise, the enhanced
momentum mixing over the warmer side of each of the SST
fronts yields anomalous wind to its downstream relative in
JRA-55CHS compared to JRA-55C. As evident in Fig. 2c, the
climatological convergence and divergence signatures tend
to accompany local increase and reduction, respectively, in
cloudiness in a manner consistent with local vertical motion.
Correspondingly, convective precipitation also tends to be
modulated in JRA-55CHS. Note that the main JRA55 product
exhibits almost the same characteristics as the JRA55C over
the western North Pacific.
ERA-Interim but only after 2001 (Masunaga et al. 2016a,b).
The enhancement is an atmospheric response to augmented
heat/moisture release from the warmer ocean with more
active warm-core eddies (Sugimoto and Hanawa 2011).
In situ observations
Though recently acknowledged as powerful means of
studying extratropical air-sea interactions, high-resolution
numerical modeling and satellite observations still involve
some uncertainties in simulating or measuring boundarylayer processes. Furthermore, their spatial and temporal
resolutions are still insufficient for fully capturing variability
of multi-scale phenomena involved in the air-sea interactions.
For further improvement of numerical modeling and
satellite observations, in situ observations are useful as
benchmarks for model simulations and ground truth for
satellite retrieval. In situ observations were conducted as a
key activity of the “Hotspot project”, as introduced below.
One of the unique in situ observations by the “Hotspot project”
was the deployment of a new mooring buoy near the KE for
eight months from June 2012 (Fig. 3a), as an addition to
two moored buoy sites within the KOE region: the Kuroshio
Extension Observatory (KEO) and Japanese KEO (JKEO),
situated to the south and north of the KE axis, respectively. The
KEO buoy at 32.3°N, 144.6°E has been operative since 2004 by
the U.S. NOAA/PMEL, whereas the JKEO buoy was operated
at 38.0°N, 146.5°E from 2007 to 2013 by the Japan Agency for
Marine-Earth Science and Technology Japan (JAMSTEC). The
New KEO (NKEO) site at 33.8°N, 144.9°E for the extra buoy
deployed was located between the KEO and JKEO sites, so as to
monitor the meridional distribution of surface air pressure, SHF
and LHF across the SST front along the KE. In reality, however,
the KE axis just east of Japan was displaced anomalously
northward in 2012, the SST difference between the KEO and
NKEO sites was less than 2°C throughout the observation
period. Furthermore, only a limited number of meteorological
variables had been monitored since September 2012, when
many meteorological sensors, including wind gauges, failed
at both the JKEO and NKEO sites, probably due to contacts
with fishing gear or vandalism. Still, SST and surface wind data
obtained at the three buoys were utilized for the validation of
AMSR2 onboard the GCOM-W satellite (Tomita et al. 2015).
Furthermore, optical dissolved oxygen (DO) sensors installed
on the mooring line of the NKEO buoy at 200, 400 and 600
m depths were successful in capturing DO changes in the
subtropical mode water (STMW) and intrusions of oxygen-rich
The aforementioned climatological local features of surface
wind convergence/divergence and associated cloudiness
represented in JRA-55C and JRA-55CHS (Fig. 2) overall
resemble those in ERA-Interim for the periods before and
after the beginning of 2002, respectively, and those features
in JRA-55CHS are more consistent with satellite observations.
Furthermore, our preliminary analysis shows that JRA-55CHS
can reproduce enhancement of cloudiness and precipitation
in the mixed-water region east of Japan during the unstable
regime of the KE jet relative to its stable regime (c.f. Qiu and
Chen 2005), as captured by satellite observations and by
a)
b)
16
18
20
22
24
Figure 3: (a) JKEO, NKEO and KEO buoy sites (dots), superimposed on map of JCOPE2 SST on 2 July 2012 (colored; every 1 °C). The R/V Natsushima
cruise in June 2012 and the coordinated three-vessel cruises in July 2012 were carried out along the pink solid and red dashed lines, respectively across
the KE front. The R/V Kaiyo cruise in July 2013 was carried out along the red solid line jointly with aircraft observations whose sites are plotted with the
closed squares. Triangular domain roughly defines the SeaGlider observation area. (b) Time-latitude section of SST observed in July 2012 by the three
vessels (°C, colored) at 143°E along the red dashed line in (a), where the observed SST values were resampled hourly and then interpolated temporally
and spatially. Red line represents the SST front. Dotted lines and dots denote the positions of the vessels and radiosonde observations, respectively.
Seisui-maru (magenta) departed right after the observation at 1500 JST on 6 July, while R/V Wakataka-maru (black) and R/V Tansei-maru (blue)
continued observations until 0700 JST on 7 July. After Kawai et al. (2015).
24
water underneath (Nagano et al. 2016). In addition, a GPS wave
gauge installed on top of the NKEO buoy was also successful
in monitoring wave height over eight months (Waseda et al.
2014). In spring of 2014, SeaGlider observations were carried
out between the KEO and S1 (30°N, 145°E) sites (Fig. 3a),
to measure vertical profiles of temperature, salinity and DO.
Acknowledgments
Another unique in situ observations by the “Hotspot project”
include an intensive observational campaign carried out in
early 2012, which was characterized by three-vessel cruises
across the KE front (Kawai et al. 2015). The three vessels,
R/V Seisui-maru, R/V Wakataka-maru and R/V Tansei-maru,
were aligned at latitudinal intervals of 30’ or 45’ along the
143°E meridian, moving back and forth meridionally within a
half-degree section across the KE front (Fig. 3b) in launching
radiosonde simultaneously every two hours. This formation
was designed to overcome the particular difficulty the
preceding single-vessel cruises encountered (Tanimoto et al.
2009; Tokinaga et al. 2009; Kawai et al. 2014) in attempting
to identify impacts of the KE front on the atmosphere. Unlike
those single-vessel cruises through which no complete
separation is possible between spatial and temporal variations
of the impacts, the three-vessel cruise made an atmospheric
section across the KE front available every four hours
throughout the 4-day period from 2 July (Kawai et al. 2015).
During that period the KE front with marked SST contrasts
displaced northward by about 50 km (Fig. 3b), which was not
well represented in any of the objectively analyzed SST data
sets available. As documented in more detail by Kawai et al.
(2015), the observations captured cross-frontal differences
in the mesoscale boundary-layer structure and their temporal
modulations due to the passage of cyclones along the Baiu
front. The bases of low-level clouds tended to be elevated over
the warmer KE water due to enhanced turbulence activity within
the developed mixed layer, and this cross-frontal contrast
became particularly prominent under the near-surface
northerlies. Likewise, joint observations with an aircraft and
R/V Kaiyo were carried out north of the KE twice in July 2013
(Fig. 3a), in an attempt to detect possible modulations of the
indirect aerosol effect on low-level clouds by the underlying
SST. In spite of low cloudiness around the targeted area, the
relationship between air-sea temperature difference and
the ratio of cloud droplet number concentration to aerosol
number concentration based on the particular observations
for summertime water clouds near the KE seems consistent
with the corresponding relationship obtained by Koike et al.
(2012) for water clouds observed over the East China Sea in
early spring. Furthermore, an aerosol-rich air mass in the free
troposphere originating from the Asian continent seemed to
be entrained into the mixed layer developing over the warm KE.
References
Concluding remarks
The “Hotspot project” was completed successfully with strong
support from the international community. Most of those
data produced and obtained through the project will become
available for the community near future. The production of
the JRA-55CHS data is under way and may become available
sometime in 2017. Comparison of JRA-55CHS with JRA-55C,
which is open to the community, will be useful for assessing
impacts of the WBCs and associated SST fronts on the
atmosphere. Since no satellite data are assimilated into either
JRA-55CHS or JRA-55C, however, their reproducibility for the
Southern Hemisphere is likely to be lower than that of ERAInterim. Most of the in situ observation data obtained during the
HotSpot project, including the NKEO buoy data, will be released
at the Hot Spot website (http://hadley1.atmos.rcast.u-tokyo.
ac.jp/hotspot), except the aircraft and SeaGlider data. A full
list of datasets available has been compiled by Cronin (2016).
25
The “Hotspot project” was conducted under the support of the
Japanese Ministry of Education, Culture, Sports, Science and
Technology through the Grants-in-Aid for Scientific Research in
Innovative Areas 2205. The aircraft observations were supported
also by Grant-in-Aid for Scientific Research (A) 26241003.
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Kita, H. Matsui, N. Oshima, M. Kajino, and T. Y. Nakajima, 2012:
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and T. Suga, 2015: “Hot spots” in the climate system –
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Taniguchi, 2016: Ventilation revealed by the observation
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26
Mesoscale Imprints
of the Kuroshio
Extension and
Oyashio Fronts on
the Wintertime
Atmospheric
Boundary Layer
Ryusuke Masunaga1, Hisashi
Nakamura1, Takafumi Miyasaka1,
Kazuaki Nishii1, Youichi Tanimoto2,
Bo Qiu3
1. Research Center for Advanced Science and
Technology, The University of Tokyo, Japan
2. Faculty of Environmental Earth Science, and
Graduate School of Environmental Science,
Hokkaido University, Sapporo, Japan
3. Department of Oceanography, University of
Hawai‘i at Mānoa, Honolulu, HI, USA
Introduction
In the Kuroshio-Oyashio Extension (KOE) region east of Japan,
also known as the subarctic oceanic frontal zone (SAFZ), part
of the cool Oyashio water flows southward along the east
coast of Japan, while the rest flows eastward to the north of
the Kuroshio Extension(KE), forming the mixed water region
in between (Kawai 1972; Yasuda 2003; Kida et al. 2015). In the
SAFZ, turbulent heat fluxes from the ocean are substantially
enhanced in the cold season under dry, cold continental air
mass advected by the prevailing monsoonal northerlies (e.g.,
Kwon et al. 2010). Despite a huge amount of heat release into
the atmosphere, SST remains relatively high in winter along
the KE owing to its advective effect. The SAFZ is recognized
as one of the major centres of action of decadal sea surface
temperature (SST) variability (Nakamura et al. 1997; Seager
et al. 2001; Nakamura and Kazmin 2003). The decadal SST
anomalies in the SAFZ can significantly modify the basin-scale
atmospheric circulation by modulating stormtrack activity
(e.g., Taguchi et al. 2012). Recent studies have revealed that
locally enhanced turbulent heat fluxes due to warm SST can
organize mesoscale structures climatologically in the marine
atmospheric boundary layer (MABL), which cannot arise
solely from atmospheric processes (e.g., Xie 2004; Small et al.
2008; Kelly et al. 2010). Locally warm SST can reduce static
stability, where the turbulent “vertical mixing effect” enhances
downward transport of wind momentum to locally accelerate
sea-surface winds (Wallace et al. 1989), modulating sea27
surface wind divergence and curl (e.g., Nonaka and Xie 2003;
Chelton et al. 2004). At the same time, local SST maxima/
minima modulate heat fluxes into the overlying atmosphere,
leading to the formation of local minima/maxima of sea-level
pressure (SLP) through “hydrostatic effect” (Lindzen and
Nigam 1987). In fact, Tanimoto et al. (2011) found a distinct
SLP trough along the KE in a long-term ship-measured
climatology in winter, which leads to local enhancement of
cloud formation and precipitation (Tokinaga et al. 2009;
Minobe et al. 2010). Tanimoto et al. (2011) also argued that
a combination of “hydrostatic effect” and “vertical mixing
effect” climatologically yields a characteristic distribution
of near-surface ageostrophic winds around the KE.
It has been pointed out that the KE fluctuates between its
stable and unstable regimes on (quasi-) decadal time scales
(e.g., Qiu and Chen 2005). In its unstable regime, eastward
transport of the KE decreases and its path tends to be more
meandering, while its stable regime is characterized by the
stronger KE jet with its more zonal path. Several studies have
documented relationship between SST anomalies associated
with the KE variability and local mesoscale MABL structures
(Iizuka 2010; Wang and Liu 2016) or large-scale atmospheric
circulation including the strength of the Aleutian low and
storm track activity (e.g., Qiu et al. 2014; Révelard et al. 2016).
This short article is a summary of Masunaga et al. (2015, 2016),
who investigated climatological mesoscale imprints of KE and
Oyashio fronts on the wintertime MABL and their modulations
associated with interannual- to decadal-scale KE variability.
Wintertime climatology
In the KOE region, part of the Oyashio flows eastward to the
north of the KE in forming two distinct SST fronts, namely
Oyashio and KE fronts (e.g., Nonaka et al. 2006), as captured
by such high-resolution satellite SST data as OISST (Reynolds
et al. 2007). To investigate mesoscale imprints of these two
SST fronts on the overlying atmosphere, we utilize the ERAInterim global atmospheric reanalysis (Dee et al. 2011),
in which the resolution of prescribed SST data has been
substantially improved since 2002. Specifically, the SST
resolution is 1.0o x 1.0o in January 1979 through December
2001, 0.5o x 0.5o in January 2002 through January 2009, and
0.05o x 0.05o since February 2009. We have confirmed that
the latter improvement exerts virtually no impacts on the
atmospheric fields represented in the forecast model with
horizontal resolution of 0.75o. In the wintertime (December
through March) climatology for the period from January
2002 through March 2014, the SST field prescribed for the
ERA-Interim resolves the KE and Oyashio fronts separately
as distinct dual peaks of SST gradient |dSST/dy| (Fig. 1b),
whereas the |dSST/dy| exhibits a broad single maximum in
its low-resolution SST period (1979 through 2001) (Fig. 1a).
In the ERA-Interim in its high-resolution SST period, surface
upward heat fluxes yield two local maxima on the warmer
flanks of these oceanic fronts, and thereby overlying MABL
is locally warmed, leading to dual local minima of SLP in its
meridionally high-pass-filtered field (hereafter hSLP) through
“hydrostatic effect”. Indeed, there are corresponding dualpeak profiles in surface wind convergence, upward motion
and MABL depth. These mesoscale features are manifested
as local maxima of total cloud amount (TCA) along both the
KE and Oyashio fronts (Fig. 1d). Precipitation, especially
convective rainfall, also exhibits local enhancement on the
warmer flanks of these oceanic fronts. In the low-resolution
SST period, by contrast, these mesoscale atmospheric
features are totally missing in correspondence to artificially
smoothed SST distribution, as represented in TCA fields (Fig.
1c). These results suggest that both the KE and Oyashio fronts
can climatologically leave mesoscale imprints on the MABL,
leading to dual-peak profiles in mesoscale atmospheric fields.
Variability
To investigate how the mesoscale atmospheric features
described above tend to be modulated due to the decadal-scale
regime changes of the KE system (e.g., Qiu and Chen 2005),
composite maps of wintertime SST and |dSST/dy| prescribed
for the ERA-Interim are constructed separately for the unstable
and stable regimes based on the KE Index (KEI) defined by Qiu
et al. (2014). Here, we focus on the period 2002-14, in which
resolution of SST data prescribed for the ERA-Interim is high
enough to represent the KE and Oyashio fronts. The KE front
along 36oN is particularly distinct in its stable regime (colored
in Fig. 2b), while it is substantially weaker in the unstable
regime (colored in Figs. 2a). The composited difference in SST
(Fig. 2c) indicates zonally elongated dipolar anomalies. In the
unstable KE regime, more warm-core eddies are detached
from the KE jet northward (Sugimoto and Hanawa 2011;
Sasaki and Minobe 2015). Therefore SST is higher than normal
to the north of the mean KE jet, and the Oyashio front located
to the north tends to be stronger. Composites of wintertime
TCA exhibit a distinct local maximum along the KE in stable
KE regime (Fig. 2h), while TCA is higher broadly over the
KOE region in the unstable regime (Fig. 2g). Their difference
is characterized as a significant enhancement of TCA right
over the mixed water region in unstable KE regime (Fig. 2i),
which accompanies anomalous surface wind convergence
(Fig. 2f). In fact, other atmospheric fields, including upward
turbulent heat fluxes, meridionally high-pass-filtered SLP and
virtual potential temperature within MABL, and convective
precipitation indicate significant composited differences over
the mixed water region. These results indicate that mesoscale
atmospheric features as imprints of the KE front are modulated
with SST variations associated with variability of KE system.
Although most of the significant anomalies are confined to the
MABL, significant anomalies in vertical motion are found to
reach the 750-hPa level.
Furthermore, we have examined the longer-term SST
variability using OISST data, which is available since 1982
winter. We found that the composited SST fields are similar
to those in Fig. 2a-b since the late 1980s, namely anomalously
warm SST localized over the mixed water region in the unstable
KE regime, and vice versa. In the mid-1980s, however, SST is
anomalously cool over the KOE region even though KE is in the
unstable regime according to the KEI, owing to anomalously
strong southward Oyashio current along the east coast of
Japan (Nakamura and Kazmin 2003; Nonaka et al. 2006). In
fact, Miyasaka et al. (2014) pointed out the change of dominant
decadal-scale SST variability over the North Pacific region in
the late 1980s. It remains to be examined how SST has been
varying in accordance with the modulation of the KE and
Oyashio current in inter-decadal time scales, for example, by
using long-term hindcast run of eddy-resolving ocean general
circulation models.
region. We consider, however, that modulations in “vertical
mixing effect” can also play an important role. Indeed,
composited anomalous eastward SST gradient (dSST/dx)
exhibits a nearly in-phase pattern with zonal component of
anomalous surface wind divergence (dus/dx). Furthermore,
vertical wind shear in MABL over the mixed water region is
significantly weakened in the unstable KE regime in association
with anomalously warm SST (See Fig. 13 of Masunaga et al.
2016). These can be indicative of the “vertical mixing effect”
operative (e.g., Hayes et al. 1989). Please refer to discussion in
section 4c of Masunaga et al. (2016) for more detail.
It will be examined in future study how the relative importance
between the two processes varies with the KE variability
to modify near-surface wind distribution by using some
diagnostics or idealized boundary layer models (e.g., Schneider
and Qiu 2015).
Summary and discussion
On the basis of the ERA-Interim we investigated the wintertime
mesoscale imprints of the KE and Oyashio fronts on the
overlying atmosphere and how they are modulated associated
with decadal-scale variability of the KE system. We have
revealed that both KE and Oyashio fronts can climatologically
leave mesoscale imprints on the overlying atmosphere, leading
to dual-peak meridional profiles of surface heat fluxes, MABL
temperature, ascent, surface wind convergence, TCA and
convective precipitation over the KOE region. Furthermore,
these mesoscale atmospheric features are significantly
modulated by SST anomalies associated with the decadalscale KE variability. These results have been verified by
using high-resolution state-of-the-art satellite data available
in this century including QuikSCAT for sea-surface wind,
MODIS (Hubanks et al. 2008) for TCA and AMSR-E for total
precipitation. These mesoscale atmospheric features are,
however, totally missing in ERA-Interim in its low-resolution
SST period. These results suggest the importance of highresolution SST data for atmospheric reanalysis to improve their
representation of mesoscale atmospheric fields. Therefore, in
the future study, it will be meaningful to use additional products
of the new Japanese reanalysis of the global atmosphere (JRA-
Mechanisms for surface wind modulations
Two major mechanisms have been proposed through which
mesoscale SST anomalies modulate sea-surface wind fields,
namely “hydrostatic effect” and “vertical mixing effect”.
We have confirmed that the composited distributions of
meridionally high-pass-filtered virtual air temperature and their
difference within the MABL can well explain the composited
hSLP anomalies through hydrostatic relationship. This result
is indicative of the “hydrostatic effect” operative under the KE
regime changes. In fact, the interannual correlation (-0.86)
is significant between winter-mean anomalies in hSLP and
surface wind convergence if averaged over the mixed water
Figure 1: Climatological-mean wintertime (December-March)
distributions of (a) SST (contoured for every 2oC) and its equatorward
gradient (|dSST/dy|) [oC (100 km)-1; colored] based on the ERA-Interim
from January 1979 through December 2001. (c) Same as in (a), but for
total cloud amount (%) in place of |dSST/dy|. (b), (d) Same as in (a)
and (c), respectively, but for the period from January 2002 through
March 2014. Red lines indicate SST fronts at which climatological-mean
|dSST/dy| maximizes in each period.
28
Figure 2: Wintertime SST (contoured for every 2oC) and its equatorward gradient (|dSST/dy|) [oC (100km)-1; shaded as indicated to the far right]
composited for the (a) unstable and (b) stable regimes of KE on the basis of the Kuroshio Extension index. (c) Composite difference [(a) - (b)] of SST
(contoured for every 0.8OC; zero contours are omitted). (d-e) Same as in (a-b), respectively, but for surface wind convergence (contoured for every 0.3
x 10-5 s-1; positive for convergence; shaded where its values are below -0.3 x 10-5 s-1) and (f) their difference (contoured for every 0.2; zero contours are
omitted). (g-h) Same as in (a-b), respectively, but for total cloud amount (contoured for every 2%; shaded where its values exceed 72%) and (i) their
difference (contoured for every 2%; zero contours are omitted). Red lines indicate the KE and Oyashio fronts. Shadings are applied in (c), (f) and (i)
where the difference is statistically significant at the 90% confidence level.
55; Kobayashi et al. 2015), in which high-resolution SST data
(0.25o x 0.25o) is used for data assimilation.
Acknowledgments
This study is supported in part by Japanese Ministry of
Education, Culture, Sports Science and Technology (MEXT)
through Grants-in-Aid for Scientific Research in 2205, 2409
(in Innovative Areas) and 25287120 and by the Japanese
Ministry of Environment through the Environment Research
and Technology Department Fund 2A1201 and 2-1503. BQ
acknowledges support from NASA Grant NNX13AE51G.
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30
Where’s the Data?
Meghan F. Cronin
NOAA Pacific Marine Environmental Laboratory,
Seattle WA USA
Introduction
CLIVAR has a very clear data policy. All CLIVAR data should
be made available freely and without restrictions, with minimal
delay (preferably also in real-time). The details of the data
policy can be found at:
http://www.clivar.org/resources/data/data-policy
In addition, many peer-reviewed journals are now instituting
data policies requiring authors to show where to access all
data (including from numerical models) used in the paper. For
the American Geophysical Union policy, see:
http://publications.agu.org/author-resource-center/
publication-policies/data-policy/. In general, an independent
group should be able to access the data used in the study and
replicate the results using the same data. Modellers also want
to test the reproducibility of these observations and results in
their numerical models.
So where are the data?
This question was asked at the recent CLIVAR-JAMSTEC
Workshop on the Kuroshio Current and Extension System:
Theory, Observations, and Ocean Climate Modelling, which
was held jointly with the CLIVAR Ocean Model Development
Panel meeting in Yokohama Japan. This article is an attempt at
a fuller answer than the one provided at the workshop.
I shall begin by describing where my data from the Kuroshio
Extension Observatory (KEO) can be accessed. I lead the
NOAA Pacific Marine Environmental Laboratory (PMEL)
Ocean Climate Stations (OCS) group, which maintains the
NOAA KEO surface mooring in the recirculation gyre south of
the Kuroshio Extension, and the NOAA Station Papa mooring
in the NE Pacific subpolar gyre. The OCS webpages are rich in
content, providing details about the moorings and sensors, the
motivation and history of the sites, publication lists, and links
to partner webpages and data sets. We have an easy to use
data display and delivery page for the observations, as well as
for air-sea fluxes computed from the data. Please explore our
website: http://www.pmel.noaa.gov/OCS/
From 2007-2013, JAMSTEC had a surface mooring (JKEO)
deployed north of the Kuroshio Extension (KE). KEO and JKEO
data have been used to investigate how the KE affects the
atmosphere through air-sea fluxes. JKEO data and metadata
can be accessed through its project website: http://www.
jamstec.go.jp/iorgc/ocorp/ktsfg/data/jkeo/index.html
Both KEO and JKEO are part of OceanSITES (http://www.
oceansites.org/), a global network of long-term, deepwater
reference stations that aim to provide full-water column,
multi-disciplinary time series at fixed locations. As with other
observational networks, OceanSITES provides one-stop
shopping for accessing all station data within its network. In
particular, OceanSITES data are served in a self-documented
common format through two Global Data Assembly Centres
(GDAC), which can be explored through a web interface:
http://dods.ndbc.noaa.gov/oceansites/ or through ftp
(ftp://ftp.ifremer.fr/ifremer/oceansites/).
31
The KEO mooring was initiated during the Kuroshio Extension
System Study (KESS), a process study, funded primarily by
the US National Science Foundation (NSF), that took place
between 2004 and 2006 with the objective of determining
the processes governing the strength and structure of the
Kuroshio Extension recirculation gyres in relation to the
meandering jet. The KESS observational array included a line
of tall profiling moorings, embedded in an array of 46 inverted
echo sounders with bottom pressures and current meters
(CPIES). These data, as well as the shipboard measurements,
Argo float data, and KEO mooring data, are all available through
the KESS website: http://uskess.whoi.edu/. As with KEO, the
KESS Argo float data are available through the PI-maintained
websites (e.g. http://www.soest.hawaii.edu/snol/) and the
Argo network website: http://www.argo.ucsd.edu/index.html.
From 2010 to 2014, the Japanese Hot-Spot in Climate System
Experiment carried out intensive ocean and atmosphere
observations to investigate how mid-latitude fronts associated
with the KE system and with the land-sea boundary affected
the climate system. Hot-Spot observations were made from
nearly every type of platform (e.g., ships, moorings, aircraft,
floats). These data are available through the JAMSTEC data
portal:http://www.godac.jamstec.go.jp/dataportal/viewer.
htm
For finding gridded data, including many satellite products, the
Asia-Pacific Data-Research Center (APDRC) of the University
of Hawaii International Pacific Research Center (IPRC)
provides an excellent starting point. See: http://apdrc.soest.
hawaii.edu/
While this answer is more full than the one provided at the
CLIVAR workshop, it is not a comprehensive list. Rather it is
intended as a helpful starting point -- a roadmap for finding
data. Please explore! I hope this will spur further dialogue and
collaborations.
Acknowledgements
This is PMEL contribution number 4456.
Summary of the
2nd Session of the
CLIVAR Ocean Model
Development Panel
(OMDP) - Extended
Meeting on Forcing
Ocean-Ice Climate
Models
Anna Pirani1, Gokhan Danabasoglu2,
Simon Marsland3
1. Université Paris Saclay, Saint-Aubin, France
and The Abdus Salam International Centre for
Theoretical Physics, Trieste, Italy
2. National Center for Atmospheric Research
(NCAR), Boulder, CO, USA
3. CSIRO Oceans and Atmosphere, Aspendale,
Australia
Introduction
The 2nd Session of the CLIVAR Ocean Model Development
Panel (OMDP) was held on 14-16 January 2016 at the Japan
Agency for Marine-Earth Science and Technology (JAMSTEC)
in Yokohama, Japan. OMDP is grateful for the organization of
the logistical aspects of the meeting by Yoshiki Komuro. The
meeting agenda, links to presentations, and further information
are available on the meeting website at http://www.clivar.org/
omdp/japan2016.
The meeting was an extended OMDP session, whereby the
invitation to participate was extended to all those directly
involved in the development of the Coordinated Ocean-ice
Reference Experiments phase II (CORE-II) and the Ocean
Model Intercomparison Project (OMIP) protocol as well as
those who have been participating in CORE-II simulations
and their analysis. Furthermore, participation was extended
beyond those who could travel to Japan in person with a live
streaming connection generously provided by the NOAA
Modeling, Analysis, Predictions, and Projections Program.
The focus of the meeting was primarily on a detailed evaluation
of the new Japanese Reanalysis (JRA-55) atmospheric
product for forcing ocean – sea-ice climate models produced
by the Japan Meteorological Agency (JMA). Presentations
and discussions included technical aspects of the JRA-55
reanalysis, the JRA-55 / OMDP collaborative evaluation that
has been on-going since early 2015, reviews of applied and /
or additional corrections; creation of a repeat-annual-cycle
forcing data set; and preliminary simulations forced with
the JRA-55 data sets. An important goal of the meeting was
to receive input from the wider ocean and climate modelling
communities participating in the CORE-II and OMIP efforts.
Forcing Ocean – Sea-ice Climate Models
The CORE framework was first introduced in Griffies et al.
(2009). The framework defines protocols for performing global
ocean – sea-ice coupled simulations forced with common
atmospheric data sets. Therefore, the most essential element
of the CORE framework is the forcing data sets, which were
developed by Large and Yeager (2004; 2009). The overarching
hypothesis of a CORE comparison project is that global ocean –
sea-ice models run under the same atmospheric state produce
qualitatively similar solutions. This hypothesis has been found
to be valid for a certain phenomena / diagnostics, but invalid
for many others. Manuscripts analyzing the solutions from
the participating models have been published in a Special
Issue of Ocean Modelling: www.sciencedirect.com/science/
journal/14635003/vsi/10PSR6J3BV4.
To date, the CORE data sets and protocol have been
collaboratively led and supported by NCAR and the NOAA
Geophysical Fluid Dynamics Laboratory (GFDL) under the
OMDP (and formerly WGOMD). While the success and
visibility of the CORE effort have been steadily increasing,
no significant new developments or maintenance of the data
sets or the protocol have occurred during the last 5-6 years.
Broad advances in ocean and climate science have highlighted
a need to advance the scientific and engineering foundations
of CORE.
To foster such progress, OMDP convened a mini-workshop on
forcing ocean and sea-ice models in Grenoble, France on 2930 January 2015 (ICPO, 2015). A major focus was a discussion
of the state-of-the-art of various reanalysis products and
efforts as well as of various surface flux data sets and satellite
products (e.g., ERA, JRA-55, ECMWF, HOAPS, etc.). Following
extensive discussions, a major outcome of the mini workshop
was the evaluation of the JRA-55 reanalysis product compared
to other products towards its adaption as the next generation
of atmospheric forcing data sets for use in the future CORE
frameworks. The JRA-55 data sets (Kobayashi et al. 2015)
cover the period from 1958 to present, featuring both high
spatial (TL319 grid) and temporal (3-hourly) resolutions.
Furthermore, they will be updated on a near-real time basis.
JRA-55 is the first comprehensive reanalysis that has covered
the last half-century since ERA-40 by ECMWF, and is the first
one to apply 4D-variational data assimilation to this period.
A JRA-55 / OMDP collaborative effort has been underway
since early 2015 to evaluate the JRA-55 data set for use in
forcing ocean – sea-ice models. The evaluation has indicated
that JRA-55 requires adjustments (bias corrections) as
done in CORE for NCEP/NCAR reanalysis and in DRAKKAR
for ECMWF reanalyses, and tests have been undertaken on
various adjustment methods. Preliminary comparisons with
in-situ observations by buoys show that surface atmospheric
elements of the adjusted data are generally improved compared
with the JRA-55 raw data. Air temperature and specific humidity
fields near coasts and islands are affected by observations on
land which are included in the JRA-55 screen-level analysis,
requiring further improvement in the adjustment method.
Several initial intercomparison efforts evaluating model
solutions obtained with the new JRA-55 product vs. some
other forcing data sets have been undertaken. These
include: i) the JMA-MRI global model forced by different
32
development versions of JRA-55 and the CORE-II forcings;
ii) Community Earth System Model (CESM) ocean and seaice component models forced with JRA-55, CORE-II, and the
NOAA/CIRES 20th Century Reanalysis (Compo et al. 2011);
iii) MOM simulations forced with JRA-55 and CORE-II; and
iv) NEMO simulations forced with JRA-55 and DRAKKAR
data sets. The OMDP meeting report (ICPO, 2016) provides
more details on the evaluation and adjustment methods and
reference datasets used.
Based on the outcomes of these evaluation efforts and
after detailed discussions, OMDP endorsed the JRA-55
activity for the development of the next version of the OMIP
experiment, while the current CORE-II forcing is used for the
version of OMIP currently underway within the Coupled Model
Intercomparison Project phase 6 (CMIP6) initiative. The JRA55 / OMDP collaborative evaluation activity will continue to
diagnose more comprehensively various issues identified in
the evaluation so far, and explore how to further improve the
application of JRA-55 to force ocean – sea-ice models. The
distribution of the dataset once ready for public release will be
done through PCMDI.
Advancing the CORE Protocol
Several aspects of the CORE protocol were reviewed. These
were identified as items that could be improved, considering
participants’ experiences with the protocol over the past 5+
years. They include revisiting the normal year forcing objectives
and expectations considering alternative strategies such as
Repeat Annual Cycle Forcing and the approach developed by
the DRAKKAR consortium. The sensitivity of CORE-II/OMIP
simulations to the CORE-II/OMIP spin-up strategy will be
revisited and tested.
In terms of salinity restoring, OMDP recommends the use
of weak restoring as the preferred choice (e.g., with a cap
of 0.5 psu). If models choose an alternative strategy, they
should document the approach used. Recommendations are
being developed on Greenland and Antarctic liquid and solid
freshwater flux estimates for future versions of OMIP. OMDP
also recommends that groups document how Mediterranean
Outflow is treated, including its salinity properties and how the
outflow water propagates in the North Atlantic.
A new CORE-II analysis was proposed to examine the role
played by the ocean’s meridional overturning circulation in the
uptake and sequestration of transient tracers from the CMIP5
and CMIP6 model simulations in comparison with the COREII simulations. First, the skill of the models’ response, when
forced by observed time series of the chlorofluorocarbon (CFC)
distributions in the atmosphere, with regards to both surface
distributions as well as vertical sections and global inventories
will be assessed. Then, what controls different uptakes in
the models and connections with overturning patterns will
be investigated. OMIP re-enforces the recommendation that
groups participating in CORE and OMIP implement CFCs and
ideal age, following the CORE-II forcing protocol.
A CORE-II data archive containing monthly-mean output files
from many of the models participated in the CORE-II effort is
provided by NCAR and can be contributed to and accessed
using Globus. The archive stores model fields on the models’
native grids, using their own variable names in NetCDF format.
The Ocean Model Intercomparison Project
(OMIP)
The CORE-II framework has reached a relatively mature state.
Specifically, it is widely recognized as the community standard
for global ocean – sea-ice simulations, and it is being adopted
33
by many groups worldwide for the evaluation of ocean and
sea-ice components of their coupled models. The CORE-II
framework and experiments are now included in the CMIP6
as an endorsed project, i.e., OMIP. OMIP aims to provide a
framework for evaluating, understanding, and improving the
ocean and sea-ice components of global climate and earth
system models contributing to the CMIP6. OMIP addresses
these aims in two complementary ways: by providing an
experimental protocol for global ocean – sea-ice models run
with a prescribed atmospheric forcing; and by providing a
protocol for ocean diagnostics to be saved as part of CMIP6.
The Panel reviewed and discussed the physical component
of the OMIP protocol design and ocean diagnostic archival
plans, to support the finalization of the Griffies et al. (2016)
OMIP overview article that provides full details on both. The
physical portion of the OMIP experimental protocol follows
that of the CORE-II experiments. We note that OMIP includes
the previously separate Ocean Carbon Model Intercomparison
Project (OCMIP). The related protocol, requested data sets,
and relevant forcing information will be detailed in a separate
publication led by James Orr of IPSL. This merging of ocean
physical, chemical, and biogeochemical efforts into a single
project allows for efficient communications across these
communities participating in CMIP6.
The OMIP diagnostic protocol is relevant for any ocean
model component of CMIP6, including the DECK (Diagnostic,
Evaluation and Characterization of Klima experiments),
historical simulations, FAFMIP (Flux Anomaly Forced MIP),
C4MIP (Coupled Carbon Cycle Climate MIP), DAMIP (Detection
and Attribution MIP), DCPP (Decadal Climate Prediction
Project), ScenarioMIP (Scenario MIP), as well as the ocean –
sea-ice OMIP simulations. The bulk of the Griffies et al. (2016)
paper offers scientific rationale for saving these diagnostics.
OMDP at the CLIVAR Open Science Conference
(OSC)
All lead authors of the CORE-II papers were solicited – and
they subsequently agreed – each prepare a poster for a COREII poster cluster at the CLIVAR OSC. OMDP will hold an open
panel meeting on Saturday (17 September 2016) in Qingdao
ahead of the OSC, focusing on the “finalization” of the JRA-55
data sets. OMDP members will also attend sessions held by
the CLIVAR GSOP, CDP, ARP, PRP, and SORP, and the DCVP
and EBUS Research Foci.
Acknowledgment
The OMDP would like to express its gratitude to Anna Pirani for
her dedication and tireless efforts as the CLIVAR Project Office
science liaison to OMDP. Anna has taken on a new position as
the head of the IPCC WG1 Technical Support Unit. She played
a critical role in the success of the OMDP (formerly Working
Group on Ocean Model Development, WGOMD) and will be
greatly missed!
References
Compo, G. P., and Co-authors, 2011: The twentieth century
reanalysis project. Q.J.R. Meteorol. Soc., 137, 1–28.
Griffies, S. M., and Co-authors, 2009: Coordinated Ocean-ice
Reference Experiments (COREs). Ocean Modell., 26, 1–46.
Griffies, S. M., and Co-authors, 2016: Experimental and
diagnostic protocol for the physical component of the CMIP6
Ocean Model Intercomparison Project (OMIP). Geosci. Model
Dev. Discuss., doi:10.5194/gmd-2016-77 (submitted).
ICPO, 2015: CLIVAR Ocean Model Development Panel (OMDP)
mini workshop on forcing ocean and sea-ice models. CLIVAR
Publication Series No. 202, 21 pp (Available from http://
www.clivar.org/documents/report-clivar-ocean-modeldevelopment-panel-omdp-mini-workshop-forcing-oceanand-sea-ice)
ICPO, 2016: Report of the 2nd Session of the CLIVAR Ocean
Model Development Panel. CLIVAR Publication Series No.
210, 15 pp (Available from http://www.clivar.org/documents/
report-2nd-session-clivar-ocean-model-development-panel)
Kobayashi, S. and Co-authors, 2015: The JRA-55 Reanalysis:
General specifications and basic characteristics. J. Meteorol.
Soc. Japan, 93, 5-48, doi: 10.2151/jmsj.2015-001.
Large, W. G., and S. G. Yeager, 2004: Diurnal to decadal global
forcing for ocean and sea-ice models: The data sets and flux
climatologies. NCAR Technical Note TN-460+STR, 105 pp.
Large, W. G., and S. G. Yeager, 2009: The global climatology
of an interannually varying air-sea flux data set. Clim. Dyn., 33,
341-364, doi:10.1007/s00382-008-0441-3.
Figure: Participants of the 2nd Session of the CLIVAR Ocean Model Development Panel (OMDP), Yokohama, Japan
34
No. 69 (Vol 20, No.1) July 2016
Editorial ................................................................................................................................................................................................ 01
CLIVAR/JAMSTEC Workshop on the Kuroshio Current and Extension System: Theory, Observations,
and Ocean Climate Modelling-The Workshop Overview and Outcomes
Yoshiki Komuro, Gokhan Danabasoglu, Simon Marsland, Xiaopei Lin, Shoshiro Minobe, Anna Pirani,
Tatsuo Suzuki, Ichiro Yasuda...................................................................................................................................................... 01
Inter-Decadal Modulations in the Dynamical State of the Kuroshio Extension System: 1905-2015
Bo Qiu, Shuiming Chen, and Niklas Schneider..................................................................................................................... 06
Variability and Mixing in the Kuroshio and Impact on Ecosystem and Climate
Ichiro Yasuda...................................................................................................................................................................................... 09
Observations of the Kuroshio Extension by an Autonomous Microstructure Float
Takeyoshi Nagai, Ryuichiro Inoue, Amit Tandon, Hidekatsu Yamazaki................................................................... 13
Observational and Numerical Study of the Frontal- and Mesoscale Air-Sea Interaction in the Kuroshio
Extension Region in the North Pacific
Xiaopei Lin, Xiaohui Ma, Zhaohui Chen, Lixiao Xu, Xue Liu, Zhao Jing, Lixin Wu, Dexing Wu, Ping Chang.... 16
Effects of Deep Bottom Topography on the Kuroshio Extension Studied by a Nested-grid OGCM
Masao Kurogi, Yukio Tanaka, and Hiroyasu Hasumi ....................................................................................................... 19
An Extra Product of the JRA-55 Atmospheric Reanalysis and In Situ Observations in the KuroshioOyashio Extension Under the Japanese “Hotspot Project”
Hisashi Nakamura, Yoshimi Kawai, Ryusuke Masunaga, Hirotaka Kamahori, Chiaki Kobayashi
and Makoto Koike............................................................................................................................................................................. 22
Mesoscale Imprints of the Kuroshio Extension and Oyashio Fronts on the Wintertime Atmospheric
Boundary Layer
Ryusuke Masunaga, Hisashi Nakamura, Takafumi Miyasaka, Kazuaki Nishii, Youichi Tanimoto,
Bo Qiu..................................................................................................................................................................................................... 27
Where’s the Data?
Meghan F. Cronin.............................................................................................................................................................................. 31
Summary of the 2nd Session of the CLIVAR Ocean Model Development Panel (OMDP) - Extended
Meeting on Forcing Ocean-Ice Climate Models
Anna Pirani, Gokhan Danabasoglu, Simon Marsland........................................................................................................ 32
The CLIVAR Exchanges is published by the International CLIVAR Project Office
ISSN No: 1026-0471
Editor: Nico Caltabiano (CLIVAR)
Guest editors: Yoshiki Komuro (JAMSTEC)
Layout: Harish J. Borse, ICMPO at IITM, Pune, India
Production, Printing and Mailing by the ICMPO with support of IITM and the Indian Ministry of Earth Sciences.
Note on Copyright:
Permission to use any scientific material (text as well as figures) published in CLIVAR Exchanges should be obtained from the
authors. The reference should appear as follows: Authors, Year, Title. CLIVAR Exchanges, No.pp. (Unpublished manuscript).
The ICPO is supported by
China State Oceanographic Administration /First Institute of Oceanography (FIO), Indian Ministry of Earth Sciences/ Indian
Institute of Tropical Meteorology (IITM) and NASA, NOAA, NSF and DoE through US CLIVAR.
WCRP is sponsored by the World Meteorological Organization,the International Council for Science and the Intergovernmental
Oceanographic Commission of UNESCO.
Contact:
Executive Director, ICPO
First Institute of Oceanography, SOA, 6 Xianxialing Road, Laoshan District, Qingdao 266061, China
[email protected]
http://www.clivar.org
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35

Exchanges 69

Transcription

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